Generate Class bytecode files with Asm

一、背景

This is actually a series,It's all about how to dynamically change code at execution time,从动态代理、到Javaassist,然后到Asm,All to do the same thing.

1、Java代理

2、Javassist动态修改Class对象

相对于Javassist,Asmis directly generated binaryClass文件,效率会更高,But writing code is particularly tedious.

二、AsmSome important class descriptions

1、ClassVistor接口的两个子类,ClassWriter和ClassReader,一个用来生成Class文件,一个用来写Class文件.If the modification already existsClass,你需要先使用ClassReader来分析Class文件.
2、MethodVistor类
visitCode：ASM开始扫描这个方法
visitMaxs(maxStack, maxLocals); This code uses at most one heap element 和一个 局部变量
visitEnd()    输出完成.
visitVarInsn：Access to local is a quantity.

三、用Asm生成HelloWorld例子

This syntax is still difficult to write,It took a long time to successfully outputHelloWorld,可以在Eclipse或IDEA中安装bytecodecan check oneClass文件的字节码,Then modify the code with reference to the bytecode file.