// attach to target VM
VirtualMachine vm = VirtualMachine.attach("2177");

// start management agent
Properties props = new Properties();
props.put("com.sun.management.jmxremote.port", "5000");
vm.startManagementAgent(props);

// detach
vm.detach();

public VirtualMachine attachVirtualMachine(String vmid)
    throws AttachNotSupportedException, IOException
{
    checkAttachPermission();

    // AttachNotSupportedException will be thrown if the target VM can be determined
    // to be not attachable.
    testAttachable(vmid);

    return new VirtualMachineImpl(this, vmid);
}

VirtualMachineImpl(AttachProvider provider, String vmid)
    throws AttachNotSupportedException, IOException
{
    super(provider, vmid);

    // This provider only understands pids
    int pid;
    try {
        pid = Integer.parseInt(vmid);
    } catch (NumberFormatException x) {
        throw new AttachNotSupportedException("Invalid process identifier");
    }

    // Find the socket file. If not found then we attempt to start the
    // attach mechanism in the target VM by sending it a QUIT signal.
    // Then we attempt to find the socket file again.
    File socket_file = new File(tmpdir, ".java_pid" + pid);
    socket_path = socket_file.getPath();
    if (!socket_file.exists()) {
        File f = createAttachFile(pid);
        try {
            sendQuitTo(pid);

            // give the target VM time to start the attach mechanism
            final int delay_step = 100;
            final long timeout = attachTimeout();
            long time_spend = 0;
            long delay = 0;

            // 省略……
        } finally {
            f.delete();
        }
    }

    // Check that the file owner/permission to avoid attaching to
    // bogus process
    checkPermissions(socket_path);

    // Check that we can connect to the process
    // - this ensures we throw the permission denied error now rather than
    // later when we attempt to enqueue a command.
    int s = socket();
    try {
        connect(s, socket_path);
    } finally {
        close(s);
    }
}

-javaagent:<jar 路径>[=<选项>]
                  加载 Java 编程语言代理，请参阅 java.lang.instrument

public class Premain {

    public static void premain(String agentArgs, Instrumentation inst) {
        inst.addTransformer(new SimpleTransformer(), true);
    }

    public static class SimpleTransformer implements ClassFileTransformer {

        @Override
        public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
                ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {

            if ("test/jvm/AttachTest".equals(className)) {
                try {
                    ClassPool pool = ClassPool.getDefault();
                    CtClass cc = pool.get("test.jvm.AttachTest");
                    CtMethod method = cc.getDeclaredMethod("attachTest");

                    // 在方法前后增加 2 行 print 语句
                    method.insertBefore("System.out.println(\"simple agent before\");");
                    method.insertAfter("System.out.println(\"simple agent after\");");

                    byte[] byteCode = cc.toBytecode();
                    // 将内存中曾经被 javassist 过的 className 对象移除
                    cc.detach();
                    return byteCode;
                } catch (NotFoundException | CannotCompileException | IOException e) {
                    e.printStackTrace();
                }
            }
            return null;
        }
    }
}

public class AttachTest {

    @Test
    public void attachTest() {
        System.out.println("attachTest start");
        try {
            TimeUnit.SECONDS.sleep(10);
        } catch (InterruptedException e) {
            // ignore
        }
        System.out.println("attachTest end");
    }
}

-javaagent:/Users/yano/code/simple-agent/target/simple-agent-1.0-SNAPSHOT.jar

simple agent before
attachTest start
attachTest end
simple agent after

public class AgentMain {
    public static void agentmain(String agentArgs, Instrumentation instrumentation) {
        instrumentation.addTransformer(new DynamicTransformer(), true);
        try {
            instrumentation.retransformClasses(Class.forName("test.jvm.AttachTest"));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static class DynamicTransformer implements ClassFileTransformer {
        @Override
        public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
                ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
            if (className.contains("test/jvm/AttachTest")) {
                try {
                    ClassPool pool = ClassPool.getDefault();
                    pool.appendClassPath(new LoaderClassPath(loader));
                    CtClass cc = pool.get("test.jvm.AttachTest");
                    CtMethod method = cc.getDeclaredMethod("simplePrint");

                    // 在方法前后增加 2 行 print 语句
                    method.insertBefore("System.out.println(\"simple agent before\");");
                    method.insertAfter("System.out.println(\"simple agent after\");");

                    return cc.toBytecode();
                } catch (NotFoundException | CannotCompileException | IOException  e) {
                    e.printStackTrace();
                }
            }
            return classfileBuffer;
        }
    }
}

@Test
public void attachDynamic() {
    System.out.println("attachDynamic start");
    int i = 0;
    while (i < 5) {
        simplePrint(i++);
        try {
            TimeUnit.SECONDS.sleep(5);
        } catch (InterruptedException e) {
            // ignore
        }
    }
}

public void simplePrint(int i) {
    System.out.println(i);
}

@Test
public void agentMain() {
    VirtualMachine.list().stream()
            .filter(vm -> vm.displayName().contains("test.jvm.AttachTest,attachDynamic"))
            .findFirst().ifPresent(vmd -> {
        try {
            VirtualMachine virtualMachine = VirtualMachine.attach(vmd.id());
            virtualMachine
                    .loadAgent("/Users/yano/code/simple-agent-main/target/simple-agent-main-1.0-SNAPSHOT.jar");
            virtualMachine.detach();
        } catch (Exception e) {
            e.printStackTrace();
        }
    });
}

attachDynamic start
0
simple agent before
1
simple agent after
simple agent before
2
simple agent after
simple agent before
3
simple agent after
simple agent before
4
simple agent after