intrace/howintraceworks.html

---
layout: default
title: How InTrace Works
---
<h1>How InTrace Works</h1>

<p>InTrace is a Java Agent which transforms Java bytecode at runtime to add instrumentation. InTrace works without access to source code but the following example will illustrate the transformation which InTrace is performing.</p>
<p>This is a short Example Java source file defining a program which runs forever making random time dependent choices.</p>
<pre>
package example;

import java.util.Arrays;

public class TraceExample
{

  /**
   * @param args
   * @throws Exception
   */
  public static void main(String[] args)
  {
    try
    {
      otherMain(args[0]);
    }
    catch (Throwable ex)
    {
      ex.printStackTrace();
    }
  }

  public static void otherMain(String arg) throws Exception
  {
    while (true)
    {
      Thread.sleep(Long.parseLong(arg));
      workMethod("foobar");
    }
  }

  private static void workMethod(String foo)
  {
    long currentTime = System.currentTimeMillis();
    System.setProperty("a", foo);
    System.setProperty("foo", exceptionMethod());
    System.setProperty("foo", ": " + intArrayMethod(new int[]
    { 1, 2, 3 }));
    if ((currentTime % 2) == 0)
    {
      System.setProperty("a", "Even time");
    }
    else
    {
      System.setProperty("a", "Odd time");
    }
  }

  private static String exceptionMethod()
  {
    try
    {
      long currentTime = System.currentTimeMillis();
      if ((currentTime % 2) == 0)
      {
        throw new Exception("Exception text");
      }
    }
    catch (Exception ex)
    {
      return "seen exception";
    }
    return "no exception";
  }

  private static int intArrayMethod(int[] intArg)
  {
    System.setProperty("a", Arrays.toString(intArg));
    return 123;
  }
}

</pre>
<p>This is what the program gets transformed to at runtime by InTrace. The source listed here is a manually cleaned up copy based on using Jadclipse to decompile the transformed bytecode produced by InTrace.</p>
<pre>
package example;

import java.util.Arrays;
import org.intrace.output.AgentHelper;

public class TraceExample
{
    public TraceExample()
    {
        AgentHelper.enter("example.TraceExample", "&lt;init&gt;", 5);
        AgentHelper.exit("example.TraceExample", "&lt;init&gt;", 5);
    }

    public static void main(String args[])
    {
       AgentHelper.enter("example.TraceExample", "main", 16);
       AgentHelper.val("Arg", "example.TraceExample", "main", args);
       try
       {
         otherMain(args[0]);
       }
       catch (Throwable ex)
       {
         AgentHelper.caught("example.TraceExample", "main", 18, ex);
         ex.printStackTrace();
       }
       AgentHelper.exit("example.TraceExample", "main", 22);
       return;
    }

    public static void otherMain(String s) throws Exception
    {
        AgentHelper.enter("example.TraceExample", "otherMain", 28);
        AgentHelper.val("Arg", "example.TraceExample", "otherMain", s);
        do
        {
            AgentHelper.branch("example.TraceExample", "otherMain", 28);
            Thread.sleep(Long.parseLong(s));
            workMethod("foobar");
        } while(true);
       // No exit trace is generated as the Java Compiler spotted this was an infinite loop
       // and did not bother to output a return instruction.
    }

    private static void workMethod(String s)
    {
        AgentHelper.enter("example.TraceExample", "workMethod", 35);
        AgentHelper.val("Arg", "example.TraceExample", "workMethod", s);
        long l = System.currentTimeMillis();
        System.setProperty("a", s);
        System.setProperty("foo", exceptionMethod());
        System.setProperty("foo", (new StringBuilder(": ")).append(intArrayMethod(new int[] {
            1, 2, 3
        })).toString());
        if(l % 2L == 0L)
        {
            AgentHelper.branch("example.TraceExample", "workMethod", 42);
            System.setProperty("a", "Even time");
        } else
        {
            AgentHelper.branch("example.TraceExample", "workMethod", 46);
            System.setProperty("a", "Odd time");
        }
        AgentHelper.exit("example.TraceExample", "workMethod", 48);
    }

    private static String exceptionMethod()
    {
        AgentHelper.enter("example.TraceExample", "exceptionMethod", 54);
        try
        {
          long l = System.currentTimeMillis();
          if(l % 2L == 0L)
          {
              AgentHelper.branch("example.TraceExample", "exceptionMethod", 57);
              throw new Exception("Exception text");
          }
       }
       catch (Exception ex)
       {
         AgentHelper.caught("example.TraceExample", "exceptionMethod", 62, ex);
         AgentHelper.val("Return", "example.TraceExample", "exceptionMethod", "seen exception");
         AgentHelper.exit("example.TraceExample", "exceptionMethod", 62);
         return "seen exception";
       }       
        AgentHelper.val("Return", "example.TraceExample", "exceptionMethod", "no exception");
        AgentHelper.exit("example.TraceExample", "exceptionMethod", 64);
        return ((String) ("no exception"));
    }

    private static int intArrayMethod(int ai[])
    {
        AgentHelper.enter("example.TraceExample", "intArrayMethod", 69);
        AgentHelper.val("Arg", "example.TraceExample", "intArrayMethod", ai);
        System.setProperty("a", Arrays.toString(ai));
        AgentHelper.val("Return", "example.TraceExample", "intArrayMethod", 123);
        AgentHelper.exit("example.TraceExample", "intArrayMethod", 70);
        return 123;
    }
}
</pre>