JVMTI
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Provides services that allow Java programming language agents to instrument programs running on the JVM. The mechanism for instrumentation is modification of the byte-codes of methods.
java agent本质上可以理解为一个jar包插件，通过JVMTI（JVM Tool Interface）完成加载，最终借助JPLISAgent（Java Programming Language Instrumentation Services Agent）完成对目标代码的修改。
Java 探针
JDK 1.5 +
Attach 方式 JDK 1.6+
- 类加载前修改字节码
- 运行时修改字节码
- 获取 类、对象 信息
包结构
- addTransformer – adds a transformer to the instrumentation engine
- getAllLoadedClasses – returns an array of all classes currently loaded by the JVM
- retransformClasses – facilitates the instrumentation of already loaded classes by adding byte-code
- removeTransformer – unregisters the supplied transformer
- redefineClasses – redefine the supplied set of classes using the supplied class files, meaning that the class will be fully replaced, not modified as with retransformClasses
Instrumentation
实现ClassFileTransformer
public class MyClassInfo {
private String className;
private List<String> methodNames;
// getter setter etc.
}
编写 MyAgent 类，premain 方法，装配 MyClassFileTransformer
配置 MyAgent，使之生效
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
<configuration>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
<archive>
<manifest>
<addClasspath>true</addClasspath>
</manifest>
<manifestEntries>
<Can-Redefine-Classes>true</Can-Redefine-Classes>
<Premain-Class>com.bage.study.agent.transform.service.MyAgent</Premain-Class>
</manifestEntries>
</archive>
</configuration>
</plugin>
打包启动
mvn package
mvn assembly:assembly
java -javaagent:transform-service-1.0-SNAPSHOT.jar -jar demo-service-1.0-SNAPSHOT.jar
运行结果
编写 MyAgent 类，agentmain 方法
配置 MyAgent，使之生效
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
<configuration>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
<archive>
<manifest>
<addClasspath>true</addClasspath>
</manifest>
<manifestEntries>
<Can-Redefine-Classes>true</Can-Redefine-Classes>
<Re-Transform-Classes>true</Re-Transform-Classes>
<Agent-Class>com.bage.study.agent.hotload.service.MyAgent</Agent-Class>
</manifestEntries>
</archive>
</configuration>
</plugin>
打包启动
mvn assembly:assembly
运行时候关联
运行效果
- 不能新增、删除、重命名属性或方法，也不能修改方法的签名
- 修改字节码：1.基于源代码修改重新编译 2.javassist等等
- jar 执行命令顺序不能更改
- 不能新加载一个不存在的class
- 慎用慎用