Instrumentation 介绍与使用

import java.lang.instrument.Instrumentation;
public class Agent {
    /**
     * 编写一个 Java 类 包含
     * public static void premain(String agentArgs, Instrumentation inst);  [1]
     * public static void premain(String agentArgs); [2]
     * @param options
     * @param ins
     */
    public static void premain(String options, Instrumentation ins) {
        if (options != null) {
            System.out.printf("  I've been called with options: \"%s\"\n", options);
        }
        else {
            System.out.println("  I've been called with no options.");
        }
        ins.addTransformer(new Transformer());
    }
}

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
import java.util.Date;
public class Transformer implements ClassFileTransformer {
    @Override
    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
        if ("com/wangxiaoming/instrument/Dog".equals(className)) {
            System.out.println("Dog's method invoke at\t" + new Date());
        }
        return null;
    }
}

Manifest-Version: 1.0
Premain-Class: com.wangxiaoming.instrument.Agent
Can-Redefine-Classes: true

<build>
        <finalName>agent</finalName>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>2.3.1</version>
                <configuration>
                    <archive>
                        <manifestFile>src/main/resources/META-INF/MANIFEST.MF</manifestFile>
                    </archive>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>2.18.1</version>
                <configuration>
                    <skipTests>true</skipTests>
                </configuration>
            </plugin>
            <plugin>
                <artifactId>maven-assembly-plugin</artifactId>
                <configuration>
                    <outputDirectory>${basedir}</outputDirectory>
                    <archive>
                        <index>true</index>
                        <manifest>
                            <addClasspath>true</addClasspath>
                        </manifest>
                        <manifestEntries>
                            <Premain-Class>com.wangxiaoming.instrument.Agent</Premain-Class>
                        </manifestEntries>
                    </archive>
                    <descriptorRefs>
                        <descriptorRef>jar-with-dependencies</descriptorRef>
                    </descriptorRefs>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>

public class InstrumentTest {
    public static void main(String[] args) {
        System.out.println(new Dog().hello());
    }
}

public class Dog {
    public String hello() {
        return "wow wow~";
    }
}

java -javaagent:jar 文件的位置 [= 传入 premain 的参数 ]

I've been called with options: "hello"
Dog's method invoke at    Sun Sep 22 23:01:52 CST 2019
wow wow~