深入了解ASM

三、ASM
我们知道Java是静态语言，而python、ruby是动态语言，Java程序一旦写好很难在运行时更改类的行为，而python、ruby可以。
不过基于bytecode层面上我们可以做一些手脚，来使Java程序多一些灵活性和Magic，ASM就是这样一个应用广泛的开源库。

ASM is a Java bytecode manipulation framework. It can be used to dynamically generate stub classes or other proxy classes,
directly in binary form, or to dynamically modify classes at load time, i.e., just before they are loaded into the Java
Virtual Machine.

ASM完成了BCEL和SERP同样的功能，但ASM
只有30多k，而后两者分别是350k和150k。apache真是越来越过气了。

让我们来看一个ASM的简单例子Helloworld.java，它生成一个Example类和一个main方法，main方法打印"Hello world!"语句:

import java.io.FileOutputStream;
import java.io.PrintStream;

import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.Type;
import org.objectweb.asm.commons.GeneratorAdapter;
import org.objectweb.asm.commons.Method;

public class Helloworld extends ClassLoader implements Opcodes {

  public static void main(final String args[]) throws Exception {

    // creates a ClassWriter for the Example public class,
    // which inherits from Object

    ClassWriter cw = new ClassWriter(0);
    cw.visit(V1_1, ACC_PUBLIC, "Example", null, "java/lang/Object", null);
    MethodVisitor mw = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null,
        null);
    mw.visitVarInsn(ALOAD, 0);
    mw.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
    mw.visitInsn(RETURN);
    mw.visitMaxs(1, 1);
    mw.visitEnd();
    mw = cw.visitMethod(ACC_PUBLIC + ACC_STATIC, "main",
        "([Ljava/lang/String;)V", null, null);
    mw.visitFieldInsn(GETSTATIC, "java/lang/System", "out",
        "Ljava/io/PrintStream;");
    mw.visitLdcInsn("Hello world!");
    mw.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println",
        "(Ljava/lang/String;)V");
    mw.visitInsn(RETURN);
    mw.visitMaxs(2, 2);
    mw.visitEnd();
    byte[] code = cw.toByteArray();
    FileOutputStream fos = new FileOutputStream("Example.class");
    fos.write(code);
    fos.close();
    Helloworld loader = new Helloworld();
    Class exampleClass = loader
        .defineClass("Example", code, 0, code.length);
    exampleClass.getMethods()[0].invoke(null, new Object[] { null });

    // ------------------------------------------------------------------------
    // Same example with a GeneratorAdapter (more convenient but slower)
    // ------------------------------------------------------------------------

    cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
    cw.visit(V1_1, ACC_PUBLIC, "Example", null, "java/lang/Object", null);
    Method m = Method.getMethod("void <init> ()");
    GeneratorAdapter mg = new GeneratorAdapter(ACC_PUBLIC, m, null, null,
        cw);
    mg.loadThis();
    mg.invokeConstructor(Type.getType(Object.class), m);
    mg.returnValue();
    mg.endMethod();
    m = Method.getMethod("void main (String[])");
    mg = new GeneratorAdapter(ACC_PUBLIC + ACC_STATIC, m, null, null, cw);
    mg.getStatic(Type.getType(System.class), "out", Type
        .getType(PrintStream.class));
    mg.push("Hello world!");
    mg.invokeVirtual(Type.getType(PrintStream.class), Method
        .getMethod("void println (String)"));
    mg.returnValue();
    mg.endMethod();
    cw.visitEnd();
    code = cw.toByteArray();
    loader = new Helloworld();
    exampleClass = loader.defineClass("Example", code, 0, code.length);
    exampleClass.getMethods()[0].invoke(null, new Object[] { null });
  }
}

我们看到上面的例子分别使用ASM的MethodVisitor和GeneratorAdapter两种方式来动态生成Example类并调用打印语句。