java agent 使用及实现代码

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;
    }
  }

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());
  }
}

<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>