使用ASM动态创建类

使用ASM动态创建类，可以不需要java源文件，或者在类被JAVA虚拟机装入内存之前，动态修改类。

如下，动态创建一个类Example.class并执行。

ClassWriter cw = new ClassWriter(0);
        cw.visit(V1_5, ACC_PUBLIC, "Example", null, "java/lang/Object", null);

        // creates a MethodWriter for the (implicit) constructor
        MethodVisitor mw = cw.visitMethod(ACC_PUBLIC,
                "<init>",
                "()V",
                null,
                null);
        // pushes the 'this' variable
        mw.visitVarInsn(ALOAD, 0);
        // invokes the super class constructor
        mw.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
        mw.visitInsn(RETURN);
        // this code uses a maximum of one stack element and one local variable
        mw.visitMaxs(1, 1);
        mw.visitEnd();

        // creates a MethodWriter for the 'main' method
        mw = cw.visitMethod(ACC_PUBLIC + ACC_STATIC,
                "main",
                "([Ljava/lang/String;)V",
                null,
                null);
        // pushes the 'out' field (of type PrintStream) of the System class
        mw.visitFieldInsn(GETSTATIC,
                "java/lang/System",
                "out",
                "Ljava/io/PrintStream;");
        // pushes the "Hello World!" String constant
        mw.visitLdcInsn("Hello world!");
        // invokes the 'println' method (defined in the PrintStream class)
        mw.visitMethodInsn(INVOKEVIRTUAL,
                "java/io/PrintStream",
                "println",
                "(Ljava/lang/String;)V");
        mw.visitInsn(RETURN);
        // this code uses a maximum of two stack elements and two local
        // variables
        mw.visitMaxs(2, 2);
        mw.visitEnd();

        // gets the bytecode of the Example class, and loads it dynamically
        byte[] code = cw.toByteArray();

        FileOutputStream fos = new FileOutputStream("d:/com/Example.class");
        fos.write(code);
        fos.close();

        Helloworld loader = new Helloworld();
        Class<?> exampleClass = loader.defineClass("Example",code, 0, code.length);

        // uses the dynamically generated class to print 'Helloworld'
        exampleClass.getMethods()[0].invoke(null, new Object[] { null });

 程序执行，打印“Hello world!”。

使用反编译工具查看CLASS文件，内容如下：

public class Example
{
  public static void main(String[] paramArrayOfString)
  {
    System.out.println("Hello world!");
  }
}