什么是agent?agent 能做什么
https://docs.oracle.com/javase/7/docs/api/java/lang/instrument/package-summary.html
An agent is deployed as a JAR file. An attribute in the JAR file manifest specifies the agent class which will be loaded to start the agent. For implementations that support a command-line interface, an agent is started by specifying an option on the command-line. Implementations may also support a mechanism to start agents some time after the VM has started. For example, an implementation may provide a mechanism that allows a tool to attach to a running application, and initiate the loading of the tool's agent into the running application. The details as to how the load is initiated, is implementation dependent.
agent作为一个独立的jar ，加在classpath 上 -javaagent:jarpath[=options] ，在JVM启动后会调用agent 里的premain函数。它的作用提供了一种机制用代码的方式让agent attach到running的程序，修改字节码运行，注意这里的修改是添加代码，所以它不会改变代码原来的执行结果。
如何启动一个agent，有两个方法：
方法一：按照如下方法修改pom.xml 执行mvn package 编译出agent的jar，启动时加"-javaagent:target/premain-agent-1.0-SNAPSHOT.jar"
<plugin>
<artifactId>maven-jar-plugin</artifactId>
<version>3.0.2</version>
<configuration>
<archive>
<manifestEntries>
<Premain-Class>test.PermainAgent</Premain-Class>
<Can-Redefine-Classes>true</Can-Redefine-Classes>
<Can-Retransform-Classes>true</Can-Retransform-Classes>
</manifestEntries>
</archive>
</configuration>
</plugin>
command: mvn package
方法二： 使用 ea-agent-loader，这样可以省去打包的步骤，直接在代码里面用agent ,当然需要在pom.xml 里面加入ea-agent-loader的依赖
<dependency>
<groupId>com.ea.agentloader</groupId>
<artifactId>ea-agent-loader</artifactId>
<version>1.0.3</version>
</dependency>
在代码里面直接调用自己编写的Agent class PermainAgent
public static void main1( String[] args ) {
AgentLoader.loadAgentClass(PermainAgent.class.getName(), null);
ATM atm = new ATM();
atm.hi();
}
一个示例显示了如何编写一个MyInstrumentationAgent ，通过修改指定class ATM 的 withdrawMoney 函数，在不修改源码的情况下，统计该函数的运行时间并打印出。
具体的代码见https://github.com/kellyzly/javaagent.
运行结果：
20-01-21 06:10:07:788 INFO main test.ATM:14 - [Application] Withdrawal operation completed in:0 seconds!
核心代码的解释：ATM#withdrawMoney调用前， 当JVM加载了ATM这个class， MyInstrumentationAgent截获了这个class,调用ATMTransformer#transform 修改了ATM#withdrawMoney方法，在方法的字节码加入以下这段
LOGGER.info(\"[Application] Withdrawal operation completed in:\" + opTime + \" seconds!
ATMTransformer#transform
@Override
public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
byte[] byteCode = classfileBuffer;
String finalTargetClassName = this.targetClassName.replaceAll("\\.", "/"); //replace . with /
if (!className.equals(finalTargetClassName)) {
return byteCode;
}
if (className.equals(finalTargetClassName) && loader.equals(targetClassLoader)) {
LOGGER.info("[Agent] Transforming class MyAtm");
try {
ClassPool cp = ClassPool.getDefault();
CtClass cc = cp.get(targetClassName);
CtMethod m = cc.getDeclaredMethod(WITHDRAW_MONEY_METHOD);
m.addLocalVariable("startTime", CtClass.longType);
m.insertBefore("startTime = System.currentTimeMillis();");
StringBuilder endBlock = new StringBuilder();
m.addLocalVariable("endTime", CtClass.longType);
m.addLocalVariable("opTime", CtClass.longType);
endBlock.append("endTime = System.currentTimeMillis();");
endBlock.append("opTime = (endTime-startTime)/1000;");
endBlock.append("LOGGER.info(\"[Application] Withdrawal operation completed in:\" + opTime + \" seconds!\");");
m.insertAfter(endBlock.toString());
byteCode = cc.toBytecode();
cc.detach();
} catch (NotFoundException | CannotCompileException | IOException e) {
System.out.println("Exception"+ e);
}
}
return byteCode;
}
