JavaAgent简介

• 关于JavaAgent的简介建议参考这篇博客，本文不做过多讲解。

方法

• 创建一个maven项目，pom文件如下（可以添加，下面是基本骨架）：

  <?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>cn.gloduck</groupId>
      <artifactId>GAagent</artifactId>
      <version>1.0</version>
  
      <dependencies>
          <!--javassist依赖-->
          <dependency>
              <groupId>org.javassist</groupId>
              <artifactId>javassist</artifactId>
              <version>3.28.0-GA</version>
          </dependency>
  
      </dependencies>
  
      <build>
          <plugins>
              <plugin>
                  <!--打包插件，可以避免修改MANIFEST.MF-->
                  <groupId>org.apache.maven.plugins</groupId>
                  <artifactId>maven-assembly-plugin</artifactId>
                  <version>3.3.0</version>
                  <configuration>
                      <descriptorRefs>
                          <descriptorRef>jar-with-dependencies</descriptorRef>
                      </descriptorRefs>
                      <archive>
                          <manifestEntries>
                              <!--下面两个设置自定义的Agent，否则-javaagent启动时会报错-->
                              <Agent-Class>cn.gloduck.agent.CostAgent</Agent-Class>
                              <Premain-Class>cn.gloduck.agent.CostAgent</Premain-Class>
                              <Can-Redefine-Classes>true</Can-Redefine-Classes>
                              <Can-Retransform-Classes>true</Can-Retransform-Classes>
                          </manifestEntries>
                      </archive>
                  </configuration>
  
                  <executions>
                      <execution>
                          <!--设置打包时候执行插件-->
                          <id>make-assembly</id>
                          <phase>package</phase>
                          <goals>
                              <goal>single</goal>
                          </goals>
                      </execution>
                  </executions>
              </plugin>
          </plugins>
      </build>
  </project>

• 创建入口Class，具体参考文章。

  package cn.gloduck.agent;
  
  import java.lang.instrument.Instrumentation;
  
  public class CostAgent {
      public static void premain(String agentArgs, Instrumentation inst) {
          System.out.println("载入耗时统计Agent成功，参数为：" + agentArgs);
          inst.addTransformer(new CostTransformer(agentArgs));
      }
  
      public static void premain(String agentArgs) {
      }
  }

• 然后就是具体的一个类，使用javassist修改字节码

  package cn.gloduck.agent;
  
  import javassist.ClassPool;
  import javassist.CtClass;
  import javassist.CtMethod;
  
  import java.io.ByteArrayInputStream;
  import java.lang.instrument.ClassFileTransformer;
  import java.lang.instrument.IllegalClassFormatException;
  import java.security.ProtectionDomain;
  
  public class CostTransformer implements ClassFileTransformer {
      private String manageClassPath;
  
      public CostTransformer(String manageClassPath) {
          this.manageClassPath = manageClassPath;
      }
  
      @Override
      public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
          if (!className.startsWith(manageClassPath)) {
              return classfileBuffer;
          }
          CtClass cl = null;
          try {
              ClassPool pool = ClassPool.getDefault();
              cl = pool.makeClass(new ByteArrayInputStream(classfileBuffer));
              for (CtMethod declaredMethod : cl.getDeclaredMethods()) {
                  // 创建一个本地变量
                  declaredMethod.addLocalVariable("start", CtClass.longType);
                  declaredMethod.insertBefore("start = System.currentTimeMillis();");
                  String methodName = declaredMethod.getLongName();
                  declaredMethod.insertAfter("System.out.println(\"" + methodName + " 耗时为: \" + (System" +
                          ".currentTimeMillis() - start));");
              }
  
              return cl.toBytecode();
          } catch (Exception e) {
  
          }
          return classfileBuffer;
      }
  }

• 然后使用mvn package进行打包。

使用

• 启动的时候在vm参数中添加：

  -javaagent:<jarpath>[=<options>]

注意点

如果出现Could not find goal 'assembly' in plugin org.apache.maven.plugins:maven-assembly-plugin: among available goals help, single则点击 Plugins - assembly - assembly:single