How to implement java agent? Share use cases of java agent

How to implement java agent? 1. To implement java agent, you need to implement the premain method; 2. There must be Premain-Class in the MANIFEST.MF file.

The technology of modifying classes and methods at the bytecode level can modify the bytecode without affecting compilation. You can understand spring's aop technology

The role of java agent

The technology to modify classes and methods at the bytecode level can be modified without affecting compilation Bytecode. Can understand spring's aop technology

How to implement

1. To implement javaagent, you need to implement the premain method

2. Must be in the MANIFEST.MF file There is Premain-Class

demo implementation

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;
        }
    }

Preloading class

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 specifies the preloading class

<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>

How to use

There are two ways to specify javaagent:
1) Use the command line: java -javaagent:XXX.jar ddd.jar
2) Use the vm option of idea options.

Related articles:

Java uses agent to implement the code example before the main method

SQL Server SQL Agent service usage tutorial summary

Related videos:

Latest Java complete video tutorial

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