Java 操作字节码 简单了解

Java 操作字节码

package mytest;


import javassist.*;

/**
 * 测试使用javassist生成一个新的类
 */
public class Client
{
	public static void main(String[] args) throws Exception
	{
		// 线程池  指定类的路径 跟名字
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.makeClass("mytest.Emp");

		//创建属性 属性添加到 类中
		CtField f1 = CtField.make("private int empno;", cc);
		CtField f2 = CtField.make("private String ename;", cc);
		cc.addField(f1);
		cc.addField(f2);

		//创建方法
		CtMethod m1 = CtMethod.make("public int getEmpno(){return empno;}", cc);
		CtMethod m2 = CtMethod.make("public void setEmpno(int empno){this.empno=empno;}", cc);
		cc.addMethod(m1);
		cc.addMethod(m2);

		//添加构造器
		CtConstructor constructor = new CtConstructor(new CtClass[]{CtClass.intType, pool.get("java.lang.String")}, cc);
		constructor.setBody("{this.empno=empno; this.ename=ename;}");
		cc.addConstructor(constructor);

		cc.writeFile("c:/myjava"); //将上面构造好的类写入到c:/myjava中
		System.out.println("生成类，成功！");
	}
}

//  用 IDEA 或者 JD-gui 将 字节码编译实现的 类 打开 反编译结果如下

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package mytest;

public class Emp {
	private int empno;
	private String ename;

	public int getEmpno() {
		return this.empno;
	}

	public void setEmpno(int var1) {
		this.empno = var1;
	}

	public Emp(int var1, String var2) {
		this.empno = this.empno;
		this.ename = this.ename;
	}
}

package mytest;

import javassist.*;

import java.lang.reflect.Method;
import java.util.Arrays;

/**
 * 测试javassist的API
 * @author ljj
 */
public class Client {
	/**
	 * 处理类的基本用法
	 * @throws Exception
	 */
	public static void test01() throws Exception{
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.get("mytest.Emp");

		byte[] bytes = cc.toBytecode();
		System.out.println(Arrays.toString(bytes));

		System.out.println(cc.getName()); //获取类名
		System.out.println(cc.getSimpleName()); //获取简要类名
		System.out.println(cc.getSuperclass()); //获得父类
		System.out.println(cc.getInterfaces()); //获得接口

	}

	/**
	 * 测试产生新的方法
	 * @throws Exception
	 */
	public static void test02() throws Exception{
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.get("mytest.Emp");

//		CtMethod m = CtNewMethod.make("public int add(int a,int b){return a+b;}", cc);

		CtMethod m = new CtMethod(CtClass.intType,"add", new CtClass[]{CtClass.intType,CtClass.intType},cc);
		m.setModifiers(Modifier.PUBLIC);
		m.setBody("{System.out.println(\"sowhat.blog.csdn.net\");return $1+$2;}");

		cc.addMethod(m);

		//通过反射调用新生成的方法
		Class clazz = cc.toClass();
		Object obj = clazz.newInstance();  //通过调用Emp无参构造器，创建新的Emp对象
		Method method = clazz.getDeclaredMethod("add", int.class,int.class);
		Object result = method.invoke(obj, 200,300);
		System.out.println(result);
	}

	/**
	 * 修改已有的方法的信息，修改方法体的内容
	 * @throws Exception
	 */
	public static void test03() throws Exception{
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.get("mytest.Emp");

		CtMethod cm = cc.getDeclaredMethod("sayHello",new CtClass[]{CtClass.intType});
		cm.insertBefore("System.out.println($1);System.out.println(\"start!!!\");");
		cm.insertAt(9, "int b=3;System.out.println(\"b=\"+b);");
		cm.insertAfter("System.out.println(\"end!!!\");");

		//通过反射调用新生成的方法
		Class clazz = cc.toClass();
		Object obj = clazz.newInstance();  //通过调用Emp无参构造器，创建新的Emp对象
		Method method = clazz.getDeclaredMethod("sayHello", int.class);
		method.invoke(obj, 300);
	}

	/**
	 * 属性的操作
	 * @throws Exception
	 */
	public static void test04() throws Exception{
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.get("mytest.Emp");

//		CtField f1 = CtField.make("private int empno;", cc);
		CtField f1 = new CtField(CtClass.intType,"salary",cc);
		f1.setModifiers(Modifier.PRIVATE);
		cc.addField(f1);

//		cc.getDeclaredField("ename");   //获取指定的属性

		//增加相应的set和get方法
		cc.addMethod(CtNewMethod.getter("getSalary", f1));;
		cc.addMethod(CtNewMethod.getter("setSalary", f1));;

	}

	/**
	 * 构造方法的操作
	 * @throws Exception
	 */
	public static void test05() throws Exception {
		ClassPool pool = ClassPool.getDefault();
		CtClass cc = pool.get("mytest.Emp");

		CtConstructor[] cs = cc.getConstructors();
		for (CtConstructor c : cs) {
			System.out.println(c.getLongName());
		}
	}
	
	// 字节码操作 实现 注解的操作
	public static void test06() throws Exception{
		CtClass cc = ClassPool.getDefault().get("mytest.Emp");
		
		Object[] all = cc.getAnnotations();
		Author a = (Author)all[0];
		String name = a.name();
		int year = a.year();
		System.out.println("name: " + name + ", year: " + year);

	}

	public static void main(String[] args) throws Exception {
		test06();
	}
}


@Author(name = "sowhat", year = 2014)
class Emp
{
	private int empno;
	private String ename;

	public void sayHello(int a)
	{
		System.out.println("sayHello," + a);
	}

	public int getEmpno()
	{
		return empno;
	}

	public void setEmpno(int empno)
	{
		this.empno = empno;
	}

	public String getEname()
	{
		return ename;
	}

	public void setEname(String ename)
	{
		this.ename = ename;
	}

	public Emp(int empno, String ename)
	{
		super();
		this.empno = empno;
		this.ename = ename;
	}

	public Emp()
	{
	}
}

@interface Author
{
	String name();

	int year();
}

javaassist