Instrumentation.setNativeMethodPrefix()

UP

NativeMethodPrefix

我们的学习对象就是下面两个方法：

public interface Instrumentation {
    // Since: 1.6
    boolean isNativeMethodPrefixSupported();
    // Since: 1.6
    void setNativeMethodPrefix(ClassFileTransformer transformer, String prefix);
}

• isNativeMethodPrefixSupported: Returns whether the current JVM configuration supports setting a native method prefix.
• setNativeMethodPrefix: When used with the ClassFileTransformer, it enables native methods to be instrumented.

示例：修改 native 方法

预期目标

我们的预期目标：对 native 方法进行 instrumentation 操作。

Since native methods cannot be directly instrumented (they have no bytecodes), they must be wrapped with a non-native method which can be instrumented.

在 java.lang.StrictMath 类当中，有一个 sin 方法，它带有 native 访问标识：

public final class StrictMath {
    public static native double sin(double a);
}

We could transform the class file (with the ClassFileTransformer during the initial definition of the class) so that this becomes:

注意：修改的时机是 initial definition。换句话说，在 load-time（加载的时候）进行 instrumentation。

public final class StrictMath {
    public static double sin(double a) {
        double val = 10.0D;
        val += $$$MyAgentWrapped$$$_sin(a);
        return val;
    }

    public static native double $$$MyAgentWrapped$$$_sin(double a);
}

Where sin becomes a wrapper for the actual native method ($$$MyAgentWrapped$$$_sin) with the appended prefix “$$$MyAgentWrapped$$$_”.

# 修改前
Java: StrictMath.sin(double a)  ---> C: Java_java_lang_StrictMath_sin(JNIEnv *env, jclass unused, jdouble d)

# 修改后
Java: StrictMath.$$$MyAgentWrapped$$$_sin(double a)  ---> C: Java_java_lang_StrictMath_sin(JNIEnv *env, jclass unused, jdouble d)

在 OpenJDK 的源码中，jdk/src/share/native/java/lang/StrictMath.c 文件中记录了 sin 方法的实现：

JNIEXPORT jdouble JNICALL
Java_java_lang_StrictMath_sin(JNIEnv *env, jclass unused, jdouble d)
{
    return (jdouble) jsin((double)d);
}

预览结果

为了对转换之后的内容有一个直观的理解，我们可以借助于下面的 StaticInstrumentation 生成一个示例：

import lsieun.asm.visitor.*;
import lsieun.utils.FileUtils;
import org.objectweb.asm.*;

import java.io.File;

public class StaticInstrumentation {
    public static void main(String[] args) {
        Class<?> clazz = StrictMath.class;
        String user_dir = System.getProperty("user.dir");
        String filepath = user_dir + File.separator +
                "target" + File.separator +
                "classes" + File.separator +
                "data" + File.separator +
                clazz.getName().replace(".", "/") + ".class";
        filepath = filepath.replace(File.separator, "/");

        byte[] bytes = dump(clazz);
        FileUtils.writeBytes(filepath, bytes);
        System.out.println("file:///" + filepath);
    }

    public static byte[] dump(Class<?> clazz) {
        String className = clazz.getName();
        byte[] bytes = FileUtils.readClassBytes(className);

        ClassReader cr = new ClassReader(bytes);
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
        ClassVisitor cv = new StrictMathVisitor(cw);

        int parsingOptions = ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES;
        cr.accept(cv, parsingOptions);
        return cw.toByteArray();
    }
}

Application

package sample;

import java.lang.management.ManagementFactory;
import java.util.concurrent.TimeUnit;

public class Program {
    public static void main(String[] args) throws Exception {
        // 第一步，打印进程 ID
        String nameOfRunningVM = ManagementFactory.getRuntimeMXBean().getName();
        System.out.println(nameOfRunningVM);

        // 第二步，倒计时退出
        int count = 600;
        for (int i = 0; i < count; i++) {
            String info = String.format("|%03d| %s remains %03d seconds", i, nameOfRunningVM, (count - i));
            System.out.println(info);

            double value = StrictMath.sin(30);
            System.out.println(value);

            TimeUnit.SECONDS.sleep(1);
        }
    }
}

Agent Jar

LoadTimeAgent

package lsieun.agent;

import lsieun.cst.Const;
import lsieun.instrument.*;
import lsieun.utils.*;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;

public class LoadTimeAgent {
    public static void premain(String agentArgs, Instrumentation inst) {
        // 第一步，打印信息：agentArgs, inst, classloader, thread
        PrintUtils.printAgentInfo(LoadTimeAgent.class, "Premain-Class", agentArgs, inst);

        // 第二步，指定要处理的类
        String className = "java/lang/StrictMath";

        // 第三步，使用 inst：添加 transformer
        ClassFileTransformer transformer = new ASMTransformer(className);
        inst.addTransformer(transformer, false);
        inst.setNativeMethodPrefix(transformer, Const.NATIVE_PREFIX);
    }
}

DynamicAgent

package lsieun.agent;

import lsieun.cst.Const;
import lsieun.instrument.*;
import lsieun.utils.*;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;

public class DynamicAgent {
    public static void agentmain(String agentArgs, Instrumentation inst) {
        // 第一步，打印信息：agentArgs, inst, classloader, thread
        PrintUtils.printAgentInfo(DynamicAgent.class, "Agent-Class", agentArgs, inst);

        // 第二步，指定要处理的类
        String className = "java.lang.StrictMath";

        // 第三步，使用 inst：进行 re-transform 操作
        ClassFileTransformer transformer = new ASMTransformer(className);
        inst.addTransformer(transformer, true);
        inst.setNativeMethodPrefix(transformer, Const.NATIVE_PREFIX);
        try {
            Class<?> clazz = Class.forName(className);
            if (inst.isModifiableClass(clazz)) {
                inst.retransformClasses(clazz);
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            inst.removeTransformer(transformer);
        }
    }
}

ASMTransformer

package lsieun.instrument;

import lsieun.asm.visitor.*;
import org.objectweb.asm.*;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
import java.util.Objects;

public class ASMTransformer implements ClassFileTransformer {
    private final String internalName;

    public ASMTransformer(String internalName) {
        Objects.requireNonNull(internalName);
        this.internalName = internalName.replace(".", "/");
    }

    @Override
    public byte[] transform(ClassLoader loader,
                            String className,
                            Class<?> classBeingRedefined,
                            ProtectionDomain protectionDomain,
                            byte[] classfileBuffer) throws IllegalClassFormatException {
        if (className.equals(internalName)) {
            System.out.println("transform class: " + className);
            ClassReader cr = new ClassReader(classfileBuffer);
            ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
            ClassVisitor cv = new StrictMathVisitor(cw);

            int parsingOptions = ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES;
            cr.accept(cv, parsingOptions);

            return cw.toByteArray();
        }

        return null;
    }
}

Run

mvn clean package

None

$ java -cp ./target/classes/ sample.Program

3284@LenovoWin7
|000| 3284@LenovoWin7 remains 600 seconds
-0.9880316240928618
...

Load-Time

$ java -cp ./target/classes/ -javaagent:./target/TheAgent.jar sample.Program

3332@LenovoWin7
|000| 3332@LenovoWin7 remains 600 seconds
transform class: java/lang/StrictMath
9.011968375907138
...

setNativeMethodPrefix

在 LoadTimeAgent 类当中，注释掉以下语句：

inst.setNativeMethodPrefix(transformer, Const.NATIVE_PREFIX);

再次运行，会遇到 UnsatisfiedLinkError 错误：

$ java -cp ./target/classes/ -javaagent:./target/TheAgent.jar sample.Program

2548@LenovoWin7
|000| 2548@LenovoWin7 remains 600 seconds
transform class: java/lang/StrictMath
Exception in thread "main" java.lang.UnsatisfiedLinkError: java.lang.StrictMath.$$$MyAgentWrapped$$$_sin(D)D
        at java.lang.StrictMath.$$$MyAgentWrapped$$$_sin(Native Method)
        at java.lang.StrictMath.sin(Unknown Source)
        at sample.Program.main(Program.java:18)

Can-x-Native-x-Prefix

将 Can-Set-Native-Method-Prefix 设置成 false，再次运行，会遇到 UnsupportedOperationException 异常：

$ java -cp ./target/classes/ -javaagent:./target/TheAgent.jar sample.Program

Exception in thread "main" java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at sun.instrument.InstrumentationImpl.loadClassAndStartAgent(InstrumentationImpl.java:386)
        at sun.instrument.InstrumentationImpl.loadClassAndCallPremain(InstrumentationImpl.java:401)
Caused by: java.lang.UnsupportedOperationException: setNativeMethodPrefix is not supported in this environment
        at sun.instrument.InstrumentationImpl.setNativeMethodPrefix(InstrumentationImpl.java:211)
        at lsieun.agent.LoadTimeAgent.premain(LoadTimeAgent.java:21)
        ... 6 more
FATAL ERROR in native method: processing of -javaagent failed

Dynamic: Unsupported

使用 Dynamic Attach 的方式来运行，会遇到 UnsupportedOperationException 异常：

transform class: java/lang/StrictMath
java.lang.UnsupportedOperationException: class redefinition failed: attempted to add a method
	at sun.instrument.InstrumentationImpl.retransformClasses0(Native Method)
	at sun.instrument.InstrumentationImpl.retransformClasses(InstrumentationImpl.java:144)
	at lsieun.agent.DynamicAgent.agentmain(DynamicAgent.java:25)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at sun.instrument.InstrumentationImpl.loadClassAndStartAgent(InstrumentationImpl.java:386)
	at sun.instrument.InstrumentationImpl.loadClassAndCallAgentmain(InstrumentationImpl.java:411)

Dynamic: replace

修改 StrictMathVisitor 类的 visitMethod() 方法：替换掉 native 方法的实现。

import lsieun.asm.cst.Const;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

import static org.objectweb.asm.Opcodes.*;

/**
 * native method instrumentation.
 */
public class StrictMathVisitor extends ClassVisitor {
    public StrictMathVisitor(ClassVisitor classVisitor) {
        super(Const.ASM_VERSION, classVisitor);
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
        boolean isNative = (access & Opcodes.ACC_NATIVE) != 0;
        if (isNative && "sin".equals(name) && "(D)D".equals(descriptor)) {
            // ---> 修改开始
            int newAccess = access & (~Opcodes.ACC_NATIVE);
            MethodVisitor mv = super.visitMethod(newAccess, name, descriptor, signature, exceptions);
            if (mv != null) {
                mv.visitCode();
                mv.visitLdcInsn(new Double("10.0"));
                mv.visitInsn(DRETURN);
                mv.visitMaxs(2, 2);
                mv.visitEnd();
            }
            return null;
            // <--- 修改结束
        }
        else {
            return super.visitMethod(access, name, descriptor, signature, exceptions);
        }
    }

}

再次运行，输出结果：

|007| 9304@LenovoWin7 remains 593 seconds
-0.9880316240928618


transform class: java/lang/StrictMath
|008| 9304@LenovoWin7 remains 592 seconds
10.0

总结

本文内容总结如下：

• 第一点，作用。setNativeMethodPrefix 方法的作用是为了对 native method 进行 instrumentation。
• 第二点，时机。对 native method 进行 instrumentation 的时机是 initial definition of the class。