java agent 使用及实现代码

2019-11-26 09:46:59

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java agent的作用

在字节码这个层面对类和方法进行修改的技术，能够在不影响编译的情况下，修改字节码。可以理解spring的aop技术

如何实现

1.实现javaagent需要实现premain方法
2.必须在MANIFEST.MF文件中有Premain-Class

demo实现

agent package com.xueyou.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;  public class FirstAgent implements ClassFileTransformer {    public final String injectedClassName = "com.xueyou.agentdemo.App";    public final String methodName = "hello";    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {      className = className.replace("/", ".");  //    System.out.println(className);      if (className.equals(injectedClassName)) {        CtClass ctclass = null;        try {          ctclass = ClassPool.getDefault().get(className);// 使用全称,用于取得字节码类<使用javassist>          CtMethod ctmethod = ctclass.getDeclaredMethod(methodName);// 得到这方法实例          ctmethod.insertBefore("System.out.println(11111111);");          return ctclass.toBytecode();        } catch (Exception e) {          System.out.println(e.getMessage());          e.printStackTrace();        }      }      return null;    }  }

预加载类

package com.xueyou.demo;import com.xueyou.demo.agent.FirstAgent;import java.lang.instrument.Instrumentation;/** * Hello world! */public class App {  public static void premain(String agentOps, Instrumentation inst) {    System.out.println("=========premain方法执行========");    System.out.println(agentOps);    // 添加Transformer    inst.addTransformer(new FirstAgent());  }}

maven指定预加载类

<build>    <plugins>      <plugin>        <groupId>org.apache.maven.plugins</groupId>        <artifactId>maven-compiler-plugin</artifactId>        <configuration>          <source>1.8</source>          <target>1.8</target>          <encoding>utf-8</encoding>        </configuration>      </plugin>      <plugin>        <groupId>org.apache.maven.plugins</groupId>        <artifactId>maven-shade-plugin</artifactId>        <version>3.0.0</version>        <executions>          <execution>            <phase>package</phase>            <goals>              <goal>shade</goal>            </goals>            <configuration>              <transformers>                <transformer                    implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">                  <manifestEntries>                    <PremainClass>com.xueyou.demo.App</Premain-Class>                  </manifestEntries>                </transformer>              </transformers>            </configuration>          </execution>        </executions>      </plugin>    </plugins>  </build>

如何使用

这里有两种方式指定javaagent：

1）使用命令行：java -javaagent:XXX.jar ddd.jar
2）使用idea的vm option选项。