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Processing Stack Trace Data in Java


September 17, 2002

Java Programming Notes # 1784


Preface

The recently released JavaTM 2 SDK, Standard Edition Version 1.4 contains a number of new features.  This article explains how to use some of those new features to get, interpret, and process stack trace data.

Prior to release 1.4, stack trace information was available only in the printed format produced by the printStackTrace method.  With the advent of the StackTraceElement class, and the getStackTrace method of the Throwable class, stack trace information is now available through programmatic access.

As a result, you can write logic into your programs to analyze and make decisions on the basis of stack trace information.  In addition, you can encapsulate that information into objects.  This makes it possible for you to serialize and write stack trace data to a disk file, send it out over a network, or perform any other action that is appropriate for information encapsulated in an object.

Viewing tip

You may find it useful to open another copy of this lesson in a separate browser window.  That will make it easier for you to scroll back and forth among the different listings and figures while you are reading about them.

Supplementary material

I recommend that you also study the other lessons in my extensive collection of online Java tutorials.  You will find those lessons published at Gamelan.com.  However, as of the date of this writing, Gamelan doesn't maintain a consolidated index of my Java tutorial lessons, and sometimes they are difficult to locate there.  You will find a consolidated index at www.DickBaldwin.com.

Discussion and Sample Code

I'm going to jump right into a sample program that illustrates the programmatic handling of stack trace data without a lot of preliminary discussion.  Along the way, I will also illustrate Java's new cause or exception chaining facility.  (An earlier article entitled Chained Exceptions in Java discusses this facility in detail.)

The program named StackTr01

A complete listing of the program named StackTr01 is provided in Listing 21 near the end of the lesson.  I will discuss the program in fragments.  However, before getting into the fragments, I will provide an overview of the operation of the program.

Five classes

The program consists of the following five classes:

  • StackTr01
  • Class01
  • NewEx01
  • NewEx02
  • NewEx03
The controlling class is named StackTr01.  Most of the work gets done in the class named Class01.

There are three new exception classes named NewEx01, NewEx02, and NewEx03.  These classes are used to instantiate exception objects, which are thrown by the methods in Class01.  Each of these three classes extends Exception.

The main method of the controlling class

The controlling class consists solely of the main method.  Code in the main method instantiates an object of type Class01 and invokes a method named meth01 on that object's reference.

Methods in Class01

The class named Class01 defines the following methods:

  • meth01
  • meth02
  • meth03
  • meth04
  • encapsulate - encapsulate stack trace data in a Vector object
  • writeSer - write a serialized Vector object to a disk file
  • readSer - read a serialized Vector object from a disk file
  • display - display the stack trace data in a Vector object
The first four methods in the list are used to create a chain of exceptions.  The remaining four methods in the list provide the behavior indicated in the text.

Method invocation sequence

As mentioned above, the method named meth01 is invoked by the main method in the controlling class.

The method named meth01 invokes meth02.  The method named meth02 invokes meth03, and the method named meth03 invokes meth04.  Each of these method invocations is contained in a try block, followed by a catch block of type Exception.

Throw an exception

The method named meth04 at the end of the invocation chain throws an exception of type NewEx03.  This exception is caught by the catch block in meth03.

The code in the catch block in meth03 encapsulates the incoming object's reference as the cause in a new exception object of type NewEx02, and throws that new exception object.

Similarly, the exception thrown by meth03 is caught by meth02.  The code in the catch block of meth02 encapsulates its incoming exception as the cause in a new exception object of type NewEx01 and throws it.  As you can see, this code creates a chain of exceptions as control moves back up the call stack.

Here is where things get interesting

Finally, the exception thrown by meth02 is caught by meth01.  Most of the interesting code is contained in the catch block for meth01, which is designed to illustrate the programmatic handling of stack trace data.

Print the stack trace

The catch block in meth01 receives an incoming exception parameter of type NewEx01.  The printStackTrace method is invoked on that object's reference to give us a baseline for comparison.

Encapsulate stack trace data in a Vector object

Then the stack trace data is extracted from the incoming exception object and encapsulated in an object of type Vector.  The Vector object is serialized and written to a disk file to illustrate one form of programmatic handling of stack trace data.  This Vector object contains the entire chain of exceptions, and the stack trace data associated with each exception in the chain.

Read and display the serialized stack tract data

Then the serialized data is read from the disk file, and a Vector object containing the original stack trace data is reconstructed.  The reconstructed object is passed to a custom print method, which extracts the information from the Vector object and displays it in a format similar to the standard format produced by the printStackTrace method.

Encapsulating stack trace data in an object

These operations illustrate that you now have the ability to extract stack trace data and encapsulate it in an object.  Then, you can do just about anything with the stack trace data that you can do with any other kind of data encapsulated in an object.  As a result, stack trace data is now available for programmatic access and handling.

The custom exception classes

Listing 1 shows the three custom exception classes.
 

class NewEx01 extends Exception{
  public NewEx01(String message,
                  Throwable throwable){
    super(message, throwable);}
}//end NewEx01
//===================================//
class NewEx02 extends Exception{
  public NewEx02(String message,
                  Throwable throwable){
    super(message, throwable);}
}//end NewEx02
//===================================//
class NewEx03 extends Exception{
  public NewEx03(String message){
    super(message);}
}//end NewEx03

Listing 1

Overloaded constructors

Some comments are in order regarding these exception class definitions.  Subsequent to release 1.4, whenever you define a new exception class, you should provide four overloaded constructors having argument lists that match the following four constructors of the Throwable class:

  • Throwable()
  • Throwable(String message)
  • Throwable(String message, Throwable cause)
  • Throwable(Throwable cause)
By making the argument lists of your constructors match those of the Throwable class, objects instantiated from your new classes will be capable of supporting the new cause or chained exception facility in release 1.4.

Did not provide four overloaded constructors

However, for purposes of this sample program, I knew that I would not need all four constructors in each class.  For brevity, I included only the constructor that I would need in each of the new exception classes.

(Note that the argument list for NewEx03 is different from the argument lists for the other two.  That is because the program does not encapsulate a cause when it instantiates and throws an object of type NewEx03.)
Pass parameters to superclass constructor

The code in each of the constructors is very simple.  In each case, the parameters received by the constructor are passed to the constructor for the superclass.  This causes the parameter information to be saved in instance variables of the Throwable class, and makes it possible later to successfully invoke the methods of the Throwable class on objects instantiated from these classes.

Traversing the call stack

The overview that I provided earlier explains that there is a sequence of method calls, which place several methods on the stack.  In other words, a chain of method calls is created on the runtime stack.

Then a method at the end of the chain named meth04 throws an exception.  That exception is caught by the method named meth03, which was the method that called meth04.  The method named meth03 encapsulates the incoming exception as a cause in another exception of a different type and throws the new exception.

Each time a new exception is thrown, one method is popped off the stack.  This catch and throw process continues until a method near the top of the stack, named meth01, provides code to handle the exception without throwing a new exception.

Begin discussion where the exception is thrown

I'm going to begin the discussion with the method named meth04 that throws the first exception.  Then I will continue back through the call stack discussing each method in sequence.

Listing 2 shows meth04, which throws the first exception.  This triggers the chain of exceptions being thrown as control progresses back through the call stack.
 

  void meth04() throws NewEx03{
    throw new NewEx03(
                 "Thrown from meth04");
  }//end meth04

Listing 2

The method named meth04 in Listing 2 constructs and throws an exception object of type NewEx03.  Note that this exception object does not encapsulate a cause.

Catch and throw

The exception object of type NewEx03 is caught by the catch block of type Exception in meth03, shown in Listing 3.
 

  void meth03() throws NewEx02{
    try{
      meth04();
    }catch(Exception e){      
      throw new NewEx02(
               "Thrown from meth03",e);
    }//end catch
  }//end meth03

Listing 3

The most important thing to note about Listing 3 is the instantiation and throwing of a new exception object of type NewEx02.  This code uses a constructor for NewEx02 that encapsulates the incoming object reference of type NewEx03 as the cause in the new object.

(Note that the declared type of the catch block in meth03 is Exception.  This catch block can catch objects instantiated from the class Exception or any of its subclasses. NewEx03 is a subclass of Exception.)
Catch and throw again

The exception object thrown by meth03 in Listing 3 is caught by the method named meth02 in Listing 4.
 

  void meth02() throws NewEx01{
    try{
      meth03();
    }catch(Exception e){
      throw new NewEx01(
               "Thrown from meth02",e);
    }//end catch
  }//end meth02

Listing 4

The code in listing 4 is very similar to that in Listing 3.  The code in meth02 instantiates and throws a new object of type NewEx01, with the incoming object of type NewEx02 encapsulated as the cause in that object.

Handling the exception

The exception thrown by meth02 in Listing 4 is caught and processed by the method named meth01, which begins in Listing 5.  The code in meth01 does not instantiate and throw a new exception object.  Rather, it completely handles the incoming exception in its catch block, thus breaking the chain of exceptions.
 

  void meth01(){
    try{
      meth02();//call meth02
    }catch(Exception e){
      System.out.println(
                   "Print StackTrace");
      e.printStackTrace();

Listing 5

Print the stack trace

The code in Listing 5 begins by invoking the printStackTrace method on the incoming parameter to cause the stack trace to be printed in the standard format on the standard error stream.  The output is shown in Figure 1 (we will need this later for comparison purposes).
 

Print StackTrace
NewEx01: Thrown from meth02
 at Class01.meth02(StackTr01.java:92)
 at Class01.meth01(StackTr01.java:60)
 at StackTr01.main(StackTr01.java:52)
Caused by: NewEx02: Thrown from meth03
 at Class01.meth03(StackTr01.java:102)
 at Class01.meth02(StackTr01.java:85)
 ... 2 more
Caused by: NewEx03: Thrown from meth04
 at Class01.meth04(StackTr01.java:112)
 at Class01.meth03(StackTr01.java:100)
 ... 3 more

Figure 1

The print format

A word about the format of the data in Figure 1 may be in order.  Those lines that read ... 2 more and ... 3 more indicate that the specified number of lines were omitted for brevity.  As far as I know, this is the standard output format of the printStackTrace method over which we have no control.  (We will see later that these lines were not omitted from my custom print format for the same stack tract data.)

Encapsulate stack trace data in a Vector object

Continuing with the catch block in meth01, the statement in Listing 6 invokes the encapsulate method to cause the stack trace data to be extracted from the incoming exception object and encapsulated in a Vector object.
 

      Vector vecOuter = encapsulate(e);

Listing 6

The encapsulate method

At this point, I'm going to set the method named meth01 aside for the moment and discuss the method named encapsulate, (which is a method of my own design).  The beginning of the encapsulate method is shown in Listing 7.  This is a rather long method, so I will discuss it in fragments.  Before getting into the details of the method, however, I will provide some background information.
 

  Vector encapsulate(Throwable e){
    Vector vecOuter = new Vector();
    Throwable cause = e;

Listing 7

Contents of the incoming Throwable object

The encapsulate method receives a reference to a Throwable object as an incoming parameter.  In this case, it is the Throwable object of type NewEx01 caught by the method named meth01.

The end of a chain of exceptions

Recall that this Throwable object was the result of a chain of operations involving methods catching objects of one type, encapsulating the incoming object as the cause in an object of another type, and then throwing the object of the new type.

Encapsulates the history of the call stack

As a result, this incoming object contains a lot of information about the history of the call stack from the point where the original exception was thrown to the end of the chain of exceptions.  In other words, the incoming Throwable object contains a Throwable cause, which contains a Throwable cause, which contains a Throwable cause, etc.

We will process the incoming Throwable object in order to extract all of that information.

What does a Throwable object contain?

According to Sun:

"A throwable contains a snapshot of the execution stack of its thread at the time it was created. It can also contain a message string that gives more information about the error. Finally, it can contain a cause: another throwable that caused this throwable to get thrown. The cause facility is new in release 1.4. It is also known as the chained exception facility, as the cause can, itself, have a cause, and so on, leading to a "chain" of exceptions, each caused by another."
Assumes a chain of exceptions

The procedure that I will use assumes that at each point along the sequence of exceptions being caught and thrown, a reference to the incoming Throwable object was encapsulated as a cause in the new object.

(This is what constitutes a chain of exceptions.  If the chain is broken by a failure of a catch block somewhere along the way to encapsulate its incoming exception as a cause, neither the printStackTrace method, nor this procedure can obtain information about what happened beyond the break in the chain.)
No cause in the original object

The original object that was thrown did not encapsulate a Throwable as a cause.  Therefore, when we reach the point where the cause encapsulated in the Throwable object equals null, we will conclude that we have reached the end of the chain.

Save stack trace data along the way

This algorithm will drill down, getting and processing Throwable (cause) objects, until it reaches the point where the getCause method returns null.  At each step along the way, it will get and save the stack trace information encapsulated in that particular Throwable object.  Each set of stack trace data will be saved in a Vector object.

Save the Vector objects in a Vector object

The set of Vector objects so produced will be saved in another Vector object.  Thus, we will end up with a Vector object containing references to other Vector objects.  Each of the secondary Vector objects will contain the stack trace data encapsulated in one of the Throwable objects encountered along the way.

Create the outer Vector object

The code in Listing 7 instantiates a new Vector object and saves it in a reference variable named vecOuter.  This Vector object will be populated with references to other Vector objects as the algorithm progresses.

Save incoming reference in variable named cause

The code in Listing 7 also saves the incoming reference in a reference variable having the descriptive name cause.  This reference variable is used in the conditional clause of the while loop that follows.

Begin a while loop

Listing 8 shows the beginning of a while loop.  This loop controls the process of drilling down to the point where a Throwable (cause) object is encountered with a cause value of null (the end of the chain of exceptions).
 

    while(cause != null){
      StackTraceElement[] trace 
               = cause.getStackTrace();

Listing 8

Get and save array of StackTraceElement objects

The code in Listing 8 invokes the getStackTrace method to get and save an array of references to StackTraceElement objects.  These objects represent the stack trace information encapsulated in the Throwable(cause) object being processed during this iteration of the while loop.

What is a StackTraceElement object?

Here is what Sun has to say about an object of type StackTraceElement:

"An element in a stack trace ... Each element represents a single stack frame. All stack frames except for the one at the top of the stack represent a method invocation. The frame at the top of the stack represents the execution point at which the stack trace was generated. Typically, this is the point at which the throwable corresponding to the stack trace was created."
Thus, each StackTraceElement in the array represents one stack frame, and the collection of StackTraceElement objects in the array represents the entire stack trace.

We will be able to correlate the information in each StackTraceElement object with source code line numbers when we examine the stack trace data later.

So, now we have an array of StackTraceElement objects

At this point, we have an array object containing references to StackTraceElement objects.  Each StackTraceElement object represents one stack frame, beginning at the point where the exception represented by this Throwable object was thrown, and ending at the bottom of the stack.

Encapsulate the data in a Vector

For convenient handling later, we will encapsulate the information in this array object in a Vector object, and encapsulate that Vector object, along with other Vector objects in another Vector object.

In addition, we will also encapsulate the type of the Throwable object, along with the message encapsulated in the Throwable object in our Vector object.

The code in Listing 9 instantiates a new Vector object to contain the data.
 

      Vector vec = new Vector();

      vec.add("Cause" 
                   + cause.toString());

Listing 9

Type of Throwable and message

The code in Listing 9 also invokes the toString method on the Throwable object and concatenates the resulting string with the word "Cause".  The string that results from the concatenation is added to in the Vector object.  We will use the word "Cause" later to identify the string containing the type of the Throwable object along with the message encapsulated in the Throwable object.

"The getMessage method is also available to get the message.  However, in this case, the toString method was preferable because it returned both the type of the object and the text of the message."
Get and save information about each stack frame

Each element in the array of StackTraceElement objects contains information about one frame of the stack.  The following four methods of the StackTraceElement class are available to access four different pieces of information describing each stack frame:

  • getClassName - Returns a String, which is the name of the class containing the execution point represented by this stack trace element.
  • getMethodName - Returns a String, which is the name of the method containing the execution point represented by this stack trace element.
  • getFileName - Returns a String, which is the name of the source file containing the execution point represented by this stack trace element.
  • getLineNumber - Returns an int, which is the line number of the source line containing the execution point represented by this stack trace element.
Iterate on the array object with a for loop

The for loop in Listing 10 iterates on the array object, extracting and saving each of the four pieces of information for each element in the array (for each frame in the stack).
 

      for(int i=0;i<trace.length;i++){
        vec.add("Class" + trace[i].
                       getClassName());
        vec.add("Method" + trace[i].
                      getMethodName());
        vec.add("File" + trace[i].
                        getFileName());
        vec.add("Line" + trace[i].
                      getLineNumber());
      }//end for loop

Listing 10

Concatenate with a descriptive word

For convenient handling later on, each of the four pieces of information is concatenated to a word that describes the type of information.

(Note that this concatenation process causes the int line number value to be converted to a string.)
Contents of the Vector object

Each of the four strings (each containing information about a single stack frame) is added to the Vector object during each iteration of the for loop.

Thus, when the entire array object has been processed, the Vector object contains one String object describing the type of Throwable being processed and the message encapsulated in that Throwable object.  In addition, the Vector object contains four String objects representing each stack frame that existed at the point in time that the exception was thrown.

(For example, if the stack contained three frames, the Vector object would contain thirteen elements, four elements representing each stack frame plus one element representing the Throwable object.)
Add the populated inner Vector to the outer Vector

This Vector object is now fully populated.  The single statement in Listing 11 adds it to the outer Vector object that serves as a container for the individual Vector objects being produced by this process.  (Note that the statement in Listing 11 is outside the for loop.)
 

      vecOuter.add(vec);

Listing 11

Get the encapsulated Throwable and do it again

Each Throwable object may encapsulate another throwable object known as the cause.

One Throwable object is processed during each iteration of the current while loop.

The code in Listing 12 invokes the getCause method on the current Throwable object to get a reference to the Throwable object (if any) that it encapsulates.  If this Throwable object doesn't encapsulate another throwable object, the getCause method returns null.
 

      cause = cause.getCause();      
    }//end while loop

    return vecOuter;
  }//end encapsulate

Listing 12

Go back to the top of the while loop

Then control returns to the top of the while loop where the new value of cause is tested for null.  If the value of cause is not null, (meaning that there is another Throwable object to be processed), another iteration of the while loop is executed to process that Throwable object.

If the value of cause is null, control exits the while loop, causing the return statement in Listing 12 to be executed.  This return statement returns a reference to the outer Vector object, which has been populated with Vector objects.  Each of the Vector objects contained in the outer Vector object contains stack trace information encapsulated by one of the Throwable objects involved in the series of chained exceptions.

Back to meth01

The return statement returns control to the method named meth01, as shown in Listing 13.
 

//continuing in meth01
      writeSer(vecOuter,"junk");
      Vector vecIn = 
               (Vector)readSer("junk");

Listing 13

Stack trace data is encapsulated in a Vector object

At this point, the full set of stack trace data for the set of chained exceptions has been encapsulated in an object of the class Vector.  Once we have the stack trace data in this form, we can do whatever we want with it.  For example, we could serialize the Vector object and write it in a disk file or send it across the network.

Demonstrate object serialization

The code in Listing 13 demonstrates this by first serializing the Vector object to a disk file, and then reading that disk file and reconstructing the Vector object in its original state.  This is accomplished by sequentially invoking two methods that I wrote named writeSer and readSer.

Once again, I will set the method named meth01 aside temporarily and discuss the two methods mentioned above.

The writeSer method

The method named writeSer is shown in Listing 14.  This method will serialize an incoming object and write the byte stream produced by serialization into a file whose name is specified by an incoming parameter of type String.  I'm not going to explain this code in detail (I have previously published lessons on object serialization on my web site).
 

  void writeSer(Object obj, 
                          String name){
    try{//to serialize the Vector obj
      ObjectOutputStream  outStream  =
               new  ObjectOutputStream(
                  new FileOutputStream(
                                name));
      outStream.writeObject(obj);
      outStream.flush();
      outStream.close();
    }catch(Exception excp){
      System.out.println(excp);
    }//end catch
  }//end writeSer

Listing 14

The key statement in Listing 14 is the invocation of the writeObject method, which performs the serialization of the Vector object and causes the byte stream to be written into a disk file.

The readSer method

The method named readSer, shown in Listing 15, will read the file containing the serialized object, reconstruct the original object, and return a reference to the reconstructed object as type Object.
 

  Object readSer(String name){
    try{
      ObjectInputStream inStream = 
                 new ObjectInputStream(
                   new FileInputStream(
                                name));
      return inStream.readObject();
       
    }catch(Exception excp){
      System.out.println(excp);
    }//end catch
    //required to satisfy compiler
    return null;
  }//end readSer

Listing 15

The key statement in Listing 15 is the invocation of the readObject method.  This method performs the reconstruction of the original Vector object using a stream of bytes as input.

Back to meth01 again

The return statement in Listing 15 returns control to meth01, as shown in Listing 16.  Listing 16 shows the remainder of meth01.
 

//continuing in meth01
      display(vecIn);
    }//end catch    
  }//end meth01

Listing 16

Display the stack trace data

The code in Listing 16 invokes a method named display that I wrote to display the stack trace data encapsulated in the Vector object.  This data is displayed in a custom format of my own design.

Obviously, at this point, you could display the stack trace data in any format that you choose.  I chose to display it in a format that resembles the standard format produced by the printStackTrace method.  This makes it easy to confirm the validity of the stack trace data encapsulated in the Vector object by comparing the two displays.

The display method

The display method begins in Listing 17.
 

  void display(Vector vecOuter){
    Enumeration enumOuter = 
                   vecOuter.elements();
    while(enumOuter.hasMoreElements()){
      Vector vecInner = (Vector)
               enumOuter.nextElement();

Listing 17

Nested Vector objects

Recall that the stack trace data is now contained in nested Vector objects.  That is to say, one Vector object serves as an outer container for several other Vector objects.  Each of the inner Vector objects contains stack trace information pertaining to one of the Throwable objects produced by the chain of exceptions and encapsulated in the final Throwable object.

Nested enumerations

I needed to display the data encapsulated in each of the inner Vector objects.  This led to a solution based on nested enumerations.  An outer loop enumerates on the outer Vector object, to extract each of the inner vector objects in sequence.

An inner loop enumerates on each of the inner Vector objects to extract, format, and display the data encapsulated in each of the inner Vector objects.

I don't plan to discuss enumeration in detail.  I have previously published tutorial lessons explaining the enumeration process on my web site.

The outer enumeration loop

The code in Listing 17 sets up the outer enumerator loop.  This loop gets a reference to one element contained in the outer Vector object during each iteration of the loop.  These elements are also Vector objects, and each of them is referred to as vecInner.

The Vector object accessed during each iteration of the outer loop contains String data describing stack trace data originally encapsulated in a Throwable object (in the chain of throwable objects).

The inner enumeration loop

The code in Listing 18 sets up an inner enumeration loop.  This loop gets a reference to one String element contained in the inner Vector object during each iteration of the loop.
 

      Enumeration enumInner = 
                   vecInner.elements();
      while(enumInner.
                    hasMoreElements()){
        String str = (String)
               enumInner.nextElement();

Listing 18

String identifiers

Recall that each of the String objects in the Vector object begins with one of the following words:

  • Cause
  • Class
  • Method
  • File
  • Line
There should be only one String object beginning with the word Cause in each Vector object.  Then there should be one String object beginning with each of the other four words for each frame that existed on the stack at the point in time that the Throwable object was constructed and thrown.

A decision tree

Although it looks complicated, the code in Listing 19 is simply a big decision tree that tests the String object to determine which of the five types of data it represents.  Then the code in Listing 19 formats the data appropriately and displays it on the screen.
 

        if(str.startsWith("Cause")){
          System.out.print(
                 str.substring("Cause".
                            length()));
        }else if(str.startsWith(
                             "Class")){
          System.out.println();
          System.out.print("  " 
               + str.substring("Class".
                            length()));
        }else if(str.startsWith(
                            "Method")){
          System.out.print("." 
              + str.substring("Method".
                            length()));
        }else if(str.startsWith(
                              "File")){
          System.out.print("(" 
                + str.substring("File".
                            length()));
        }else if(str.startsWith(
                              "Line")){
          System.out.print(":" 
                + str.substring("Line".
                      length()) + ")");
        }//end else

Listing 19

An example output

For example, the first five passes through the inner enumeration loop on an individual Vector object might produce something like that shown in Figure 2.
 

NewEx01: Thrown from meth02
  Class01.meth02(StackTr01.java:92)

Figure 2

The first line in Figure 2 results from the String object that begins with the word Cause.  The second line is a composite of the information extracted from four String objects beginning with the words Class, Method, File, and Line.

What does the output represent?

The first line of output represents the type of one Throwable object, and the message encapsulated in that object.

The second line of output represents one stack frame that existed at the point in time that the Throwable object was created and thrown.  If the stack contained more than one frame, other lines similar to the second line would be produced, one for each stack frame.

Wrap up the display method

Listing 20 shown the remaining code in the display method, including the ends of the inner and outer enumeration loops.
 

      }//end inner while loop
      System.out.println();//blank line
    }//end outer while loop
  }//end display

Listing 20

The full output

Figure 3 shows the full output produced by the display method in this program.
 

NewEx01: Thrown from meth02
  Class01.meth02(StackTr01.java:92)
  Class01.meth01(StackTr01.java:60)
  StackTr01.main(StackTr01.java:52)
NewEx02: Thrown from meth03
  Class01.meth03(StackTr01.java:102)
  Class01.meth02(StackTr01.java:85)
  Class01.meth01(StackTr01.java:60)
  StackTr01.main(StackTr01.java:52)
NewEx03: Thrown from meth04
  Class01.meth04(StackTr01.java:112)
  Class01.meth03(StackTr01.java:100)
  Class01.meth02(StackTr01.java:85)
  Class01.meth01(StackTr01.java:60)
  StackTr01.main(StackTr01.java:52)

Figure 3

Throwable objects in the chain of exceptions

From this output, we can see that three separate Throwable objects were thrown in the chain of exceptions.  The first object that was thrown and the state of the stack at the time it was created and thrown are identified by the boldface text at the bottom of Figure 3.

As shown by the top boldface line of text, the first object that was thrown was of type NewEx03.  The message reads: "Thrown from meth04".

Five frames on the stack

There were five frames on the stack at the time the object was thrown.  According to Sun:

"All stack frames except for the one at the top of the stack represent a method invocation. The frame at the top of the stack represents the execution point at which the stack trace was generated. Typically, this is the point at which the throwable corresponding to the stack trace was created."
The frame at the bottom of the stack represents the main method of the StackTr01 class.

The frame at the top of the stack represents the method named meth04 of the class named Class01.  This is the method that threw the first exception.  The line number of 112 "represents the execution point at which the stack trace was generated."

When was the Throwable object created?

The stack trace data indicates that the object was created at line 112 in the source code for the program.  Lines 112 and 113 in the source code read as shown in Figure 4.
 

112 throw new NewEx03(
113             "Thrown from meth04");

Figure 4
(Although the statement that created the Throwable object is spread across two lines, it begins on line 112.)
Method invocations

As indicated in the earlier quotation from Sun, the remaining line numbers in the boldface text represent method invocations in the methods and classes indicated.

For example, the statements corresponding to the line numbers in the last four lines in Figure 3 are shown in Figure 5.  As you can see, each of these statements is a method invocation.
 

100  meth04();//in meth03
 85  meth03();//in meth02
 60  meth02();//in meth01
 52  new Class01().meth01();//in main

Figure 5

Compare with printed stack trace data

At this point, I recommend that you compare Figure 3 with Figure 1.  Figure 1 shows the output from the printStackTrace method for this program.  Figure 3 shows my programmatic formatting of stack trace data for the same program.  Except for the fact that some of the data is missing in Figure 1, you should be able to match up each of the data elements in Figure 3 with the corresponding data elements in Figure 1.

Programmatic handling of stack trace data

Now you know how to get stack trace information and encapsulate it in an object suitable for processing under program control.  You know how to serialize that data so that it can be written in a disk file or transmitted across a network.  You know how to read the serialized byte stream and reconstruct the original object containing the stack trace data.

Equally important, you also know how to interpret the stack trace data that is available in this fashion.  Obviously, you can't write code to process the stack trace data, and make decisions on the basis of that data, unless you know how to interpret it.

Run the Program

If you haven't already done so, I encourage you to copy the code from Listing 21 into your text editor, compile it, and execute it.  Experiment with it, making changes, and observing the results of your changes.

Remember, however, that you must be running Java version 1.4 or later to compile and execute this program.

Complete Program Listing

A complete listing of the program discussed in this lesson is shown in Listing 21.
 
/*File StackTr01.java  
Copyright 2002, R. G. Baldwin
Illustrates programmatic handling of
stack trace data for the case where
the stack trace includes causal 
information.

Tested using JDK 1.4.0 under Win2000

The output produced by the program is
similar to the following:

Print StackTrace
NewEx01: Thrown from meth02
 at Class01.meth02(StackTr01.java:92)
 at Class01.meth01(StackTr01.java:60)
 at StackTr01.main(StackTr01.java:52)
Caused by: NewEx02: Thrown from meth03
 at Class01.meth03(StackTr01.java:102)
 at Class01.meth02(StackTr01.java:85)
 ... 2 more
Caused by: NewEx03: Thrown from meth04
 at Class01.meth04(StackTr01.java:112)
 at Class01.meth03(StackTr01.java:100)
 ... 3 more
NewEx01: Thrown from meth02
  Class01.meth02(StackTr01.java:92)
  Class01.meth01(StackTr01.java:60)
  StackTr01.main(StackTr01.java:52)
NewEx02: Thrown from meth03
  Class01.meth03(StackTr01.java:102)
  Class01.meth02(StackTr01.java:85)
  Class01.meth01(StackTr01.java:60)
  StackTr01.main(StackTr01.java:52)
NewEx03: Thrown from meth04
  Class01.meth04(StackTr01.java:112)
  Class01.meth03(StackTr01.java:100)
  Class01.meth02(StackTr01.java:85)
  Class01.meth01(StackTr01.java:60)
  StackTr01.main(StackTr01.java:52)
**************************************/

import java.io.*;
import java.util.*;
import java.util.logging.*;

class StackTr01{
  public static void main(
                        String[] args){
      new Class01().meth01();
  }//end main
}//end StackTr01
//===================================//

class Class01{
  void meth01(){
    try{
      meth02();//call meth02
    }catch(Exception e){
      System.out.println(
                   "Print StackTrace");
      e.printStackTrace();
      //Encapsulate stack information
      // in a Vector
      Vector vecOuter = encapsulate(e);
      
      //Write the Vector to disk as a
      // serialized object for demo
      writeSer(vecOuter,"junk");
      //Read the serialized data and
      // reconstruct the Vector object
      Vector vecIn = 
               (Vector)readSer("junk");
      //Display the stack trace data
      // in the Vector object
      display(vecIn);
    }//end catch    
  }//end meth01
  //---------------------------------//
  
  void meth02() throws NewEx01{
    try{
      meth03();
    }catch(Exception e){
      //Construct and throw a new
      // exception object with the
      // exception caught by this
      // method encapsulated as the
      // cause
      throw new NewEx01(
               "Thrown from meth02",e);
    }//end catch
  }//end meth02
  //---------------------------------//
  
  void meth03() throws NewEx02{
    try{
      meth04();
    }catch(Exception e){      
      throw new NewEx02(
               "Thrown from meth03",e);
    }//end catch
  }//end meth03
  //---------------------------------//
  
  void meth04() throws NewEx03{
    //Construct and unconditionally
    // throw a new exception object
    // with no encapsulated cause
    throw new NewEx03(
                 "Thrown from meth04");
  }//end meth04
  //---------------------------------//
  
  //Method to encapsulate stack 
  // information in a Vector 
  // containing refs to other Vector 
  // objects
  Vector encapsulate(Throwable e){
    Vector vecOuter = new Vector();
    //Treat the incoming Throwable
    // as a cause
    Throwable cause = e;
    //Drill down to the point where
    // there is no cause encapsulated
    // in the cause
    while(cause != null){
      //Get the StackTraceElement for
      // this cause
      StackTraceElement[] trace 
               = cause.getStackTrace();
      //Create a Vector to contain
      // data from this
      // StackTraceElement
      Vector vec = new Vector();
      //Include high-level information
      // about this cause in the vector
      vec.add("Cause" 
                   + cause.toString());
      //Loop, get, and save four pieces
      // of data for each item in the
      // StackTraceElememt.  Each piece
      // of data is saved as a String
      // with an identifier prepended.
      for(int i=0;i<trace.length;i++){
        vec.add("Class" + trace[i].

                       getClassName());
        vec.add("Method" + trace[i].
                      getMethodName());
        vec.add("File" + trace[i].
                        getFileName());
        vec.add("Line" + trace[i].
                      getLineNumber());
      }//end for loop
      //Add this Vector object to the
      // outer enclosing vector
      vecOuter.add(vec);
      //Continue drilling down. Get the
      // cause encapsulated in this
      // cause and start over.  Exit
      // the loop when getCause returns
      // null
      cause = cause.getCause();      
    }//end while loop
    return vecOuter;
  }//end encapsulate
  //---------------------------------//
  
  //Method to serialize the Vector and
  // write it to a disk file.
  void writeSer(Object obj, 
                          String name){
    try{//to serialize the Vector obj
      ObjectOutputStream  outStream  =
               new  ObjectOutputStream(
                  new FileOutputStream(
                                name));
      outStream.writeObject(obj);
      outStream.flush();
      outStream.close();
    }catch(Exception excp){
      System.out.println(excp);
    }//end catch
  }//end writeSer
  //---------------------------------//
  
  //Method to read the serialized data,
  // reconstruct, and return the Vector
  // object
  Object readSer(String name){
    try{
      ObjectInputStream inStream = 
                 new ObjectInputStream(
                   new FileInputStream(
                                name));
      return inStream.readObject();
       
    }catch(Exception excp){
      System.out.println(excp);
    }//end catch
    //required to satisfy compiler
    return null;
  }//end readSer
  //---------------------------------//
  
  //Method to display the stack trace
  // data encapsulated in the Vector
  // in a specific format
  void display(Vector vecOuter){
    Enumeration enumOuter = 
                   vecOuter.elements();
    while(enumOuter.hasMoreElements()){
      Vector vecInner = (Vector)
               enumOuter.nextElement();
      Enumeration enumInner = 
                   vecInner.elements();
      while(enumInner.
                    hasMoreElements()){
        String str = (String)
               enumInner.nextElement();
        if(str.startsWith("Cause")){
          System.out.print(
                 str.substring("Cause".
                            length()));
        }else if(str.startsWith(
                             "Class")){
          System.out.println();
          System.out.print("  " 
               + str.substring("Class".
                            length()));
        }else if(str.startsWith(
                            "Method")){
          System.out.print("." 
              + str.substring("Method".
                            length()));
        }else if(str.startsWith(
                              "File")){
          System.out.print("(" 
                + str.substring("File".
                            length()));
        }else if(str.startsWith(
                              "Line")){
          System.out.print(":" 
                + str.substring("Line".
                      length()) + ")");
        }//end else
      }//end while loop
      System.out.println();
    }//end while loop
  }//end display
  //---------------------------------//
}//end Class01
//===================================//

//Note:  For brevity, I included only
// the required constructors in these
// new exception classes.
//This is a new exception class
class NewEx01 extends Exception{
  public NewEx01(String message,
                  Throwable throwable){
    super(message, throwable);
  }
}//end NewEx01
//===================================//

//This is a new exception class
class NewEx02 extends Exception{
  public NewEx02(String message,
                 Throwable throwable){
    super(message, throwable);
  }
}//end NewEx02
//===================================//

//This is a new exception class
class NewEx03 extends Exception{
  public NewEx03(String message){
    super(message);
  }
}//end NewEx03

Listing 21


Copyright 2002, Richard G. Baldwin.  Reproduction in whole or in part in any form or medium without express written permission from Richard Baldwin is prohibited.

About the author

Richard G. Baldwin is a college professor (at Austin Community College in Austin, TX) and private consultant whose primary focus is a combination of Java, C#, and XML. In addition to the many platform and/or language independent benefits of Java and C# applications, he believes that a combination of Java, C#, and XML will become the primary driving force in the delivery of structured information on the Web.

Richard has participated in numerous consulting projects, and he frequently provides onsite training at the high-tech companies located in and around Austin, Texas.  He is the author of Baldwin's Programming Tutorials, which has gained a worldwide following among experienced and aspiring programmers. He has also published articles in JavaPro magazine.

Richard holds an MSEE degree from Southern Methodist University and has many years of experience in the application of computer technology to real-world problems.

baldwin@DickBaldwin.com

-end-
 

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