For CIL / MSIL, I can write the code in a text editor and compile / decompile with ilasm / ildasm.
I can use Reflector to see the CIL generated by a .NET class.
In the Java world, javap -c shows the disassembled byte code.
How do I compile Java bytecode? (i.e. the Java equivalent of ilasm / ildasm).
Is there an IDE that supports Java bytecode?
Does the IDE support debugging i.e. single stepping / breakpoints etc.?
Bytecode Outline plugin for Eclipse
to play with bytecode you can use ASM or BCEL
Take a look at org.apache.bcel.util.BCELifier, it takes a given class and converts it to a BCEL program (in Java, of course). It will show you how certain code is generated using BCEL.
Jasmin:
http://jasmin.sourceforge.net/
It's an assembler for Java bytecode. While an above poster pointed out that hand-coding Java bytecode may not be very useful, Jasmin has been used as a backend for compilers targeting the JVM as a runtime. Thus, your compiler can output Jasmin assembler instructions and then Jasmin converts them into Java classes.
来源：https://stackoverflow.com/questions/791600/java-bytecode-equivalents-for-ilasm-ildasm