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Department of Computer Science


Version 3.1.3, June 2007

Every other CPU has a machine level debugger/viewer.  Why not one for the Java Virtual Machine?


The JVMViewer simulates JVM (Java Virtual Machine)  instructions of a method in a class.  It will show you how each instruction in a method is interpreted. Well almost.  Please review the caveats below.

Its primary purpose is to debug simple programs written with the Jasmin Assembler to show what actually is happening.  It also shows the execution of (most) methods of a compiled Java program.


  1. Installing  and running the JVMViewer
  2. Loading a class file
  3. Selecting a method
  4. Selecting parameters
  5. Stepping through
  6. Caveats
  7. References, source code
  8. Changes

Installing and running the JVMViewer

  1. Download jvmem.jar and save it in the directory where you normally keep your .class files.

    Be sure that file is saved as jvmem.jar. Some browsers/OS's  will want to save it as  If that happens, just rename it to jvmem.jar.

  2. In Windows, or Unix/Linux, bring up the Dos Command Prompt / UNIX Terminal and change directory to the directory containing your  .class files. Start the JVMViewer by entering

  3.           java -jar jvmem.jar 
              java -cp jvmem.jar jvmem.JVMViewer

    The JVMViewer is found in the jvmem package contained in the jvmem.jar file. 

    The window should look like

pic 2

Problems starting up? This program needs Java 5.0, the one with generics. If you are using 1.4, download version 2.0.1.
  1. You can start the JVMViewer in other ways, depending upon your mood.
  2. You can change the size of the display font by selecting Zoom Text or Unzoom Text from the Help submenu.


Loading a class file

     In the File submenu, Open the  .class file that you want to view.  You can browse to any directory that contains a .class file.  While clicking on a .class file icon is preferred, you can type in a class file name with or without the .class suffix.
The class file will be verified by the JVM loader.  Since there is no guarantee that a jasmin program is well behaved, and our interpreter assumes that it is, this step is necessary. (Adding a float and an integer is not a good idea.)  The display will look like

pic 3

    If you want to to load a class in a package,  you must first get to the directory where the package subdirectory is located.  Then type in  packageName.ClassName.   For example, if from the directory myprogs you can execute java myPackage.myClass, then, to load this class, you would enter in  myPackage.myClass for the file name while in the directory myprogs.

    You can see all the methods, the constant pool, and a hexadecimal dump of the class by selecting Show Class from the Method submenu.

Selecting a method

From the Method submenu, select a method.  You can distinguish between static methods and instance methods, since the latter are highlighted.  The screen will then show the object code, the stack size, and the maximum number of local variables.  This is shown in the beginning of the first figure.

Selecting Parameters 

  1. Instance methods must have an object associated with it.  The JVMViewer will create an object; it will be passed in as the first parameter.  Note: at this time, objects must have the default constructor (the one with no parameters).

  2. Required data for the parameters, if any, can be passed by selecting from the Parameters submenu.  A dialog will prompt you for the type of parameter. 

    1. For the primitive types int, char, float, long, double and boolean, you should have no problem passing in data. (The other primitive types can be easily implemented.  But  why?)

    2. For the String type, just type in the text box.  Note that pressing OK without entering any data will be the empty string "", not null.   If you want to input null, just type null.  (But expect exceptions to occur in your methods.)

    3. For object types, a message will confirm that the object has been created.  That is, if the class for the object has the default constructor (the one with no parameters).  It will complain if it cannot create such an object.  Thus, we cannot interpret methods that pass an File object (at least for this version).

    4. Arrays of  int[]char[], float[]String[], and AnyObject[] can also be passed as parameters.  An array of strings will be single words.  For an array of objects, a dialog box will ask for the size of the array.  The class for the object must have the default constructor. Pressing OK without entering any array data will create an array with 0 elements.  If you want to input null, just type null.

      The String[] parameter allows us to execute main methods.  Try it.
  1. Once all the parameters have  been filled in, the interpreter will begin execution.  The display will be similar to the first figure. The  program counter (pc) will be a at location 0 of the instructions.  The instruction to be executed is displayed. The stack (any data above ----------------) will be empty.  The values of the parameters will be shown in the first local variables.  Integers and floats are displayed as such.  Characters and booleans are converted to their integer values.  Longs and doubles take up two local variables; the higher variable value is shown as --. References to arrays and objects are shown as -> array/object.  Any local variable or stack value that is either undefined or contains null is shown as --.
The JVMViewer will complain if it cannot pass a particular parameter type.

Stepping Through

  1. Press the Step button to execute a machine instruction.  After execution, the next instruction to be executed is shown.  The stack will be shown above above ---------------- and the values of the local variables will be shown below.

  2. The default is to execute one machine instruction at a time, though you may change that value.  In particular you can choose one after another in which the interpreter continuously executes an instruction about once per second.  You can stop this by changing the step size to a number; in particular, selecting Execute no instructions will stop execution.

  3. You can set a breakpoint by entering -> <n> where <n> is the location of an instruction. For example, if you want to execute a program up to the instruction at location 100, enter -> 100. After pressing Step, the JVMViewer will execute instructions up to the breakpoint. Though you can set many breakpoints, only the one that is displayed will be active.

  4. If a method calls another method in the same class, you can Step into the called method. Another window will open with that method. (This can be fun when doing recursion.)

  5. When the method completes by one of the returns, the interpreter will show the value returned, if any.  The null value is shown as --
  1. Most, but not all,  instructions can be interpreted.  To see which ones that aren't, run
          java -cp jvmem.jar jvmem.Instructions none
    This will list the unimplemented instructions, such as the ever popular instruction  monitorenter. If you want to see the list of instructions that are implemented, run
        java -cp jvmem.jar jvmem.Instructions
  2. If a method needs an object, say MyObject, then MyObject must have the null (MyObject()) constructor.
  3. Some "unusual" cases are not yet interpreted. For example, calling a static method with a short parameter. The JVMViewer will let you know about this.
  4. Interpreting the constructors (init methods) may be quirky.
References and source code
  1. This all started with the desire to write assembly code for the Java Virtual Machine.  Looking at architectures may be fine, but writing code at a low level is an eye-opener. 
    1. For the assembler, look at the jasmin site, or download  here.
    2. An overview of the machine instructions from the Jasmine site. Detailed explanations of the machine instructions from Sun's site. The original (now archived) Jasmine reference has a better interface.
    3. Learning to write simple code for the JVM is easy.  Take a look at my  tutorial.  For more details, you can look at my notes for the course that I taught at Montclair State.
  2. Source code, in a zip file.  All files are in the package jvmem, a subdirectory of the project directory jvmming.  Unzip, compile and create javadocs, if you wish.  You'll find interesting comments. (You can see them in Version 2.1.1.)
Changes and corrections

Carl E. Bredlau
Department of Computer Science
College of Science and Mathematics
Montclair State University
1 Normal Avenue
Montclair, NJ  07043