spring-instrument源码阅读

¶spring-instrument源码阅读

¶spring-instrument模块功能

1. instrument提供了允许java通过代理服务来检测运行在jvm上的程序，可以对方法的字节码进行修改，实现虚拟机级别的aop
2. instrument功能是在java se5中加入的，instrument要求在运行前用命令行或系统参数来设置代理类.也就是在运行前设置代理类
3. 在java se6中加入了动态控制的代理的能力
4. 监控和控制虚拟机的行为。

¶asm与instrument

1. asm与instrument一样，都能操作字节码。但是instrument是在整个虚拟机上挂了一个钩子函数，每次装入一个新类，都必须执行这个函数，如果是用在改造类上，会造成很大的资源浪费。instrument更适合与监控和控制虚拟机的行为。
2. asm和instrument的侧重点不同，asm侧重改造字节码，实现动态代理;instrument侧重监控和控制虚拟机。

¶spring-instrument代码结构

¶spring-instrument源码

¶InstrumentationSavingAgent

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.instrument;

import java.lang.instrument.Instrumentation;

/**
 * Java agent that saves the {@link Instrumentation} interface from the JVM
 * for later use.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 2.0
 * @see org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver
 */

/**
 * https://www.cnblogs.com/yelao/p/9841810.html
 * https://blog.csdn.net/jiangtianjiao/article/details/88093825
 * https://www.ibm.com/developerworks/cn/java/j-lo-jpda2/index.html
 * instrument提供了允许java通过代理服务来检测运行在jvm上的程序，可以对方法的字节码进行修改，实现虚拟机级别的aop
 * instrument功能是在java se5中加入的，instrument要求在运行前用命令行或系统参数来设置代理类.也就是在运行前设置代理类
 * 在java se6中加入了动态控制的代理的能力
 *
 * 下面的实现就是为spring项目提供一个java动态代理的接口
 */
public final class InstrumentationSavingAgent {

	private static volatile Instrumentation instrumentation;


	private InstrumentationSavingAgent() {
	}

	/**
	 * 下面这两个方法都是直接使用的jdk中的instrument
	 */

	/**
	 * Save the {@link Instrumentation} interface exposed by the JVM.
	 * 这个方法是在程序运行前进行代理,也就是在main函数运行之前的一些操作(main执行之前的修改)
	 */
	public static void premain(String agentArgs, Instrumentation inst) {
		instrumentation = inst;
	}

	/**
	 * Save the {@link Instrumentation} interface exposed by the JVM.
	 * 这个方法是在运行时动态代理(控制类运行的行为)
	 * This method is required to dynamically load this Agent with the Attach API.
	 */
	public static void agentmain(String agentArgs, Instrumentation inst) {
		instrumentation = inst;
	}

	/**
	 * Return the {@link Instrumentation} interface exposed by the JVM.
	 * <p>Note that this agent class will typically not be available in the classpath
	 * unless the agent is actually specified on JVM startup. If you intend to do
	 * conditional checking with respect to agent availability, consider using
	 * {@link org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver#getInstrumentation()}
	 * instead - which will work without the agent class in the classpath as well.
	 * @return the {@code Instrumentation} instance previously saved when
	 * the {@link #premain} or {@link #agentmain} methods was called by the JVM;
	 * will be {@code null} if this class was not used as Java agent when this
	 * JVM was started or it wasn't installed as agent using the Attach API.
	 * @see org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver#getInstrumentation()
	 *
	 * 不管是使用运行前代理还是动态代理，都会产生一个instrumentation对象，这个方法就是为spring项目提供这个对象
	 */
	public static Instrumentation getInstrumentation() {
		return instrumentation;
	}

}

¶文献引用

1. AOP之利器：ASM介绍
2. 冷门instrument包，功能d炸天