背景描述
javaagent是在JDK5之后提供的新特性，也可以叫java代理。开发者通过这种机制(Instrumentation)可以在加载class文件之前修改方法的字节码(此时字节码尚未加入JVM)，动态更改类方法实现AOP，提供监控服务如；方法调用时长、可用率、内存等。
开发简述
通过实现ClassFileTransformer接口方法，动态更改方法的字节码。在方法前后加上时间戳，最后执行完成输出执行时长。
环境准备
1、IntelliJ IDEA Community Edition 2018.3.1 x64
2、jdk1.8 64位
配置信息（路径相关修改为自己的）
1、java调试时配置
2.1、配置位置：Run/Debug Configurations ->VM options
2.2、配置内容(编译后的jar放到根目录下)：-javaagent:E:\itstack-demo-javaagent-1.0-SNAPSHOT.jar=agentargs
代码示例
itstack-demo-javaagent
├── pom.xml
└── src
└── main
├── java
│ └── org.itstack.demo.agent
│ ├── MyAgent.java
│ └── MyTransformer.java
├── resources
│ ├── META-INF
│ │ └── MANIFEST.MF
│ └── log4j2.xml
└── test
└── java
└── org.itstack.demo.test
└── AgentTest.java
pom.xml
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.jd.jr.test</groupId>
<artifactId>baitiao-test-javaagent</artifactId>
<version>1.0-SNAPSHOT</version>
<name>test-javaagent</name>
<description>test-javaagent</description>
<properties>
<!-- Build args -->
<argline>-Xms512m -Xmx512m</argline>
<skip_maven_deploy>false</skip_maven_deploy>
<updateReleaseInfo>true</updateReleaseInfo>
<project.build.sourceEncoding>utf-8</project.build.sourceEncoding>
<maven.test.skip>true</maven.test.skip>
<maven.configuration.manifestFile>src/main/resources/META-INF/MANIFEST.MF</maven.configuration.manifestFile>
</properties>
<dependencies>
<dependency>
<groupId>org.javassist</groupId>
<artifactId>javassist</artifactId>
<version>3.22.0-GA</version>
</dependency>
</dependencies>
<build>
<sourceDirectory>src/main/java</sourceDirectory>
<resources>
<resource>
<directory>src/main/resources</directory>
<filtering>true</filtering>
<includes>
<include>**/**</include>
</includes>
</resource>
</resources>
<testSourceDirectory>src/test/java</testSourceDirectory>
<testResources>
<testResource>
<directory>src/test/resources</directory>
<filtering>true</filtering>
<includes>
<include>**/**</include>
</includes>
</testResource>
</testResources>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jar-plugin</artifactId>
<version>2.4</version>
<configuration>
<archive>
<manifestFile>${maven.configuration.manifestFile}</manifestFile>
</archive>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-resources-plugin</artifactId>
<version>2.5</version>
<configuration>
<encoding>${project.build.sourceEncoding}</encoding>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.2</version>
<configuration>
<source>1.6</source>
<target>1.6</target>
<encoding>${project.build.sourceEncoding}</encoding>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-source-plugin</artifactId>
<version>2.1.2</version>
<executions>
<execution>
<id>attach-sources</id>
<goals>
<goal>jar</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
<profiles>
<profile>
<id>production</id>
<activation>
<activeByDefault>true</activeByDefault>
<jdk>1.6</jdk>
</activation>
<properties>
<maven.compiler.source>1.6</maven.compiler.source>
<maven.compiler.target>1.6</maven.compiler.target>
<maven.compiler.compilerVersion>1.6</maven.compiler.compilerVersion>
</properties>
</profile>
</profiles>
</project>
MyAgent.java
package org.itstack.demo.agent;
import java.lang.instrument.Instrumentation;
public class MyAgent {
//JVM 首先尝试在代理类上调用以下方法
public static void premain(String agentArgs, Instrumentation inst) {
inst.addTransformer(new MyTransformer());
}
//如果代理类没有实现上面的方法，那么 JVM 将尝试调用该方法
public static void premain(String agentArgs) {
}
}
MyTransformer.java
package org.itstack.demo.agent;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.CtNewMethod;
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class MyTransformer implements ClassFileTransformer {
private final static String prefix = "\nlong startTime = System.currentTimeMillis();\n";
private final static String postfix = "\nlong endTime = System.currentTimeMillis();\n";
// 被处理的方法列表
private final static Map<String, List<String>> methodMap = new HashMap<String, List<String>>();
public MyTransformer() {
//对指定方法监控
add("org.itstack.demo.test.AgentTest.queryUserAge");
add("org.itstack.demo.test.AgentTest.queryUserName");
}
private void add(String methodString) {
String className = methodString.substring(0, methodString.lastIndexOf("."));
String methodName = methodString.substring(methodString.lastIndexOf(".") + 1);
List<String> list = methodMap.get(className);
if (list == null) {
list = new ArrayList<String>();
methodMap.put(className, list);
}
list.add(methodName);
}
@Override
public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
className = className.replace("/", ".");
// 判断加载的class的包路径是不是需要监控的类
if (methodMap.containsKey(className)) {
CtClass ctclass = null;
try {
ctclass = ClassPool.getDefault().get(className);// 使用全称,用于取得字节码类<使用javassist>
for (String methodName : methodMap.get(className)) {
String outputStr = "\nSystem.out.println(\"监控信息(执行耗时)：" + className + "." + methodName + " => \" +(endTime - startTime) +\"毫秒\");";
CtMethod ctmethod = ctclass.getDeclaredMethod(methodName);// 得到这方法实例
String newMethodName = methodName + "$new";// 新定义一个方法叫做比如queryUserAge$new
ctmethod.setName(newMethodName);// 将原来的方法名字修改
// 创建新的方法，复制原来的方法，名字为原来的名字
CtMethod newMethod = CtNewMethod.copy(ctmethod, methodName, ctclass, null);
// 构建新的方法体
StringBuilder methodBodyStr = new StringBuilder();
methodBodyStr.append("{");
methodBodyStr.append(prefix);
methodBodyStr.append(newMethodName + "($$);\n");// 调用原有代码，类似于method();($$)表示所有的参数
methodBodyStr.append(postfix);
methodBodyStr.append(outputStr);
methodBodyStr.append("}");
newMethod.setBody(methodBodyStr.toString());// 替换新方法
ctclass.addMethod(newMethod); // 增加新方法
}
return ctclass.toBytecode();
} catch (Exception e) {
e.printStackTrace();
}
}
return null;
}
}
MANIFEST.MF
Manifest-Version: 1.0 Premain-Class: org.itstack.demo.agent.MyAgent Can-Redefine-Classes: true
AgentTest.java
package org.itstack.demo.test;
import java.util.logging.Logger;
/**
* vm options = -javaagent:E:\itstack-demo-javaagent-1.0-SNAPSHOT.jar=agentargs
*/
public class AgentTest {
private static Logger logger = Logger.getLogger("AgentTest");
public static void main(String[] args) {
String userId = "100001";
queryUserAge(userId);
queryUserName(userId);
}
private static void queryUserAge(String userId) {
try {
Thread.sleep(300);
logger.info("hello userId：" + userId +" age 18");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private static void queryUserName(String userId) {
try {
Thread.sleep(100);
logger.info("hello userId：" + userId +" name agent");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
测试结果
2019-4-17 11:34:33 org.itstack.demo.test.AgentTest queryUserAge$new 信息: hello userId：100001 age 18 监控信息(执行耗时)：org.itstack.demo.test.AgentTest.queryUserAge => 316毫秒 2019-4-17 11:34:33 org.itstack.demo.test.AgentTest queryUserName$new 信息: hello userId：100001 name agent 监控信息(执行耗时)：org.itstack.demo.test.AgentTest.queryUserName => 100毫秒
