字节码编程bytebuddy之通过Advice动态修改方法参数值

写在前面

本文看下如何通过bytebuddy的advice切面技术来动态修改方法入参值。

1：程序

首先定义premain：

package com.dahuyou.change.method.param;

//import net.bytebuddy.agent.builder.AgentBuilder;
import net.bytebuddy.agent.builder.AgentBuilder;
import net.bytebuddy.asm.Advice;
import net.bytebuddy.description.type.TypeDescription;
import net.bytebuddy.dynamic.DynamicType;
import net.bytebuddy.matcher.ElementMatchers;
import net.bytebuddy.utility.JavaModule;
import java.lang.instrument.Instrumentation;

public class MyPreMain {

    public static void premain(String agentArgs, Instrumentation instrumentation) {
        System.out.println("MyPreMain.premain agentArgs is: " + agentArgs);

        AgentBuilder agentBuilder = new AgentBuilder.Default();

        AgentBuilder.Transformer transformer = (builder, typeDescription, classLoader, javaModule) -> {
            builder = builder.visit(
                    Advice.to(MyByteBuddyAdvice.class)
                            .on(ElementMatchers.named("newSelectorManager")));
/*
        AgentBuilder.Transformer transformer = (builder, typeDescription, classLoader, javaModule) -> {
            builder = builder.visit(
                    Advice.to(MyByteBuddyAdvice.class)
                            .on(ElementMatchers.isMethod()
                                    .and(ElementMatchers.any())
                                    .and(ElementMatchers.not(ElementMatchers.nameStartsWith("main")))));
*/
            return builder;
        };

        agentBuilder = agentBuilder.type(ElementMatchers.nameStartsWith("com.dahuyou.change.method.param")).transform(transformer).asDecorator();

        //监听
        AgentBuilder.Listener listener = new AgentBuilder.Listener() {
            @Override
            public void onDiscovery(String s, ClassLoader classLoader, JavaModule javaModule, boolean b) {

            }

            @Override
            public void onTransformation(TypeDescription typeDescription, ClassLoader classLoader, JavaModule javaModule, boolean b, DynamicType dynamicType) {
                System.out.println("onTransformation：" + typeDescription);
            }

            @Override
            public void onIgnored(TypeDescription typeDescription, ClassLoader classLoader, JavaModule javaModule, boolean b) {

            }

            @Override
            public void onError(String s, ClassLoader classLoader, JavaModule javaModule, boolean b, Throwable throwable) {

            }

            @Override
            public void onComplete(String s, ClassLoader classLoader, JavaModule javaModule, boolean b) {

            }

        };
        // 将bytebuddy的插桩逻辑安装到instrument
        agentBuilder.with(listener).installOn(instrumentation);
    }

    /**
     * 有public static void premain(String agentArgs, Instrumentation instrumentation)这个方法，本方法不会被调用
     * @param agentArgs
     */
    public static void premain(String agentArgs) {}
}

重点是代码Advice.to(MyByteBuddyAdvice.class)指定了切面类MyByteBuddyAdvice，在方法进入和方法退出时分别调用对应注解的方法，源码如下：

package com.dahuyou.change.method.param;

import net.bytebuddy.asm.Advice;

public class MyByteBuddyAdvice {

    @Advice.OnMethodEnter()
    public static void enter(@Advice.Argument(value=2, readOnly=false) int param2,
                             @Advice.Origin("#t") String className,
                             @Advice.Origin("#m") String methodName) {
//        System.out.println("MyByteBuddyAdvice.enter className " + className + ", methodName is: " + methodName);
        System.out.println("MyByteBuddyAdvice.enter param2 " + param2);
        param2 = 9090;
    }

    @Advice.OnMethodExit()
    public static void exit(@Advice.Origin("#t") String className, @Advice.Origin("#m") String methodName) {
//        System.out.println("MyByteBuddyAdvice.exit className " + className + ", methodName is: " + methodName);
    }
}

注解@Advice.OnMethodEnter()标注的方法在方法执行前调用，注解@Advice.OnMethodExit()标注的方法在方法退出前调用。@Advice.Argument(value=2, readOnly=false) int param2注解用于获取方法的第三个参数（要保证有第三个参数，并且类型是int，如果不是，我也没验证会发生什么，但肯定程序就跑不了了），#t，#m分别获取类名称和方法名称。

在enter方法中修改第三个参数的值param2 = 9090。

测试类：

//
//  ========================================================================
//  Copyright (c) 1995-2022 Mort Bay Consulting Pty Ltd and others.
//  ------------------------------------------------------------------------
//  All rights reserved. This program and the accompanying materials
//  are made available under the terms of the Eclipse Public License v1.0
//  and Apache License v2.0 which accompanies this distribution.
//
//      The Eclipse Public License is available at
//      http://www.eclipse.org/legal/epl-v10.html
//
//      The Apache License v2.0 is available at
//      http://www.opensource.org/licenses/apache2.0.php
//
//  You may elect to redistribute this code under either of these licenses.
//  ========================================================================
//

package com.dahuyou.change.method.param;

public class ServerConnector {

    public static void main(String[] args) {
        ServerConnector serverConnector = new ServerConnector();
        serverConnector.newSelectorManager(null, null, 9);
    }

    protected void newSelectorManager(Object executor, Object scheduler, int selectors) {
        showSelector(selectors);
    }

    private void showSelector(int selectors) {
        System.out.println("selector is: " + selectors);
    }
}