Bytecode, JVM, JIT, Classpath

General Information

Bytecode

% javap -c HelloWorld
Compiled from HelloWorld.java
class HelloWorld extends java.lang.Object {
    HelloWorld();
    public static void main(java.lang.String[]);
}

Method HelloWorld()
   0 aload_0
   1 invokespecial #6 >Method java.lang.Object()<
   4 return

Method void main(java.lang.String[])
   0 getstatic #7 >Field java.io.PrintStream out<
   3 ldc #1 >String "Hello World!"<
   5 invokevirtual #8 >Method void println(java.lang.String)<
   8 return

Class Path

Bytecode Verification

Binary dump of class file: dump (od -t x1 -t a HelloWorld.class).

4 bytes  magic number
4 bytes  minor version, major version
2 bytes  count of items in constant pool
constant pool array
2 bytes  access flags mask
2 bytes  index into the constant pool for this class
2 bytes  index into the constant pool for super class
2 bytes  count of direct super interfaces
interface array
2 bytes count of methods
methods array
2 bytes count of attributes
attribute array

A bad class would not normally be formed on purpose, but it might be created malicoiusly altered. We an editor we can change the bytes and see what happens.

A properly formed class file begins with the "magic" number OxCAFEBABE in the first four bytes, has the number of bytes it says it does, all the internal arrays have the proper form, all indices are within bounds etc.

• File name matches class name

        Exception in thread "main" java.lang.NoClassDefFoundError: WrongName (wrong name: HelloWorld)
        

• Begins with "magic number" OxCAFE 0xBABE

        Exception in thread "main" java.lang.ClassFormatError: HelloWorld (Bad magic number)
        

• Extra bytes (-verify required)

        Exception in thread "main" java.lang.ClassFormatError: HelloWorld (Extra bytes at the end of the class file)
        

• Variables are initialized before they are used
• No violating access restrictions on fields and methods
• Local variable accesses fall within the runtime stack
• Runtime stack does not overflow

The verifier also checks:

• final classes are not subclassed and final methods are not overriden
• all field references and method references have valid access

• all branch instructions are within bounds
• operand stack does not overflow
• all opcodes have appropriately typed arguments on the stack and in the local variable array

If we modify the bytecode of HelloWorld.class in various ways and try to run it, the JVM detects it can rejects the class file.

% java -verify HelloWorld
Exception in thread "main" java.lang.ClassFormatError: HelloWorld (Bad magic number)

% java -verify HelloWorld
Exception in thread "main" java.lang.NoClassDefFoundError: jawa/lang/Object

% java -verify HelloWorld
Exception in thread "main" java.lang.VerifyError:
(class: HelloWorld, method: main signature: ([Ljava/lang/String;)V)
Incompatible object argument for function call

Controlling a JIT

You may wish to just interpret the bytecode (and not compile) when developing an application in order to get line numbers in stack traces. In this way you can find the course of exceptions easier.

• Use the -D command-line option to set java.compiler to NONE.

        % java -Djava.compiler=NONE Application
        C:\> java -Djava.compiler=NONE Application
        

• Set the environment variable JAVA_COMPILER to NONE, using the setenv command.

        % setenv JAVA_COMPILER NONE
        C:\> set JAVA_COMPILER=NONE
        

      % setenv JAVA_COMPILER foo
      C:\> set JAVA_COMPILER=foo
      

      % java -Djava.compiler=foo myapp
      C:\> java -Djava.compiler=foo myapp
      

The letters lib are prepended to "foo" and an .so filename extension is added, so that the virtual machine will search for a JIT compiler named libfoo.so. The search for the alternative compiler is made in the jre/lib/sparc directory and or in the path specified by the sun.boot.library.path property, if it is set. Here is an example using of using the sun.boot.library.path property to specify an alternative JIT path (the example assumes a JIT named libfoo.so is located in directory /java):

      java -Dsun.boot.libary.path=/jdk1.2.2/jre/lib/sparc:/java -Djava.compiler=foo MyApp
      

Under Windows the library is foo.dll.