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如何使用Java字节码插装工具？

  • Post category:Java

使用Java字节码插装工具的过程可以分为以下几个步骤：

  1. 编写插件代码
  2. 生成插件jar包
  3. 配置项目依赖
  4. 运行目标程序，并加载插件

接下来将会逐一讲解每一个步骤。

1. 编写插件代码

Java字节码插装工具可以通过修改字节码来为目标程序添加功能或者改变其行为。因此，我们需要先编写一个插件代码，具体实现可以参考Java字节码编程相关的知识。插件代码可以通过以下命令行参数来进行调试：

java -javaagent:path/to/your/agent.jar your.MainClass

2. 生成插件jar包

将插件代码编译成jar包，并将其打包进一个独立的jar包中，该jar包可以放置在任意位置。生成jar包的方式可以采用maven或者gradle等构建工具，具体内容可参考相关文档。

3. 配置项目依赖

将生成的jar包配置为目标程序的依赖项，以便在程序运行时能够正确加载插件。例如，如果使用maven构建目标程序，可以将以下代码添加到pom文件中：

<dependencies>
    <dependency>
        <groupId>your.group.id</groupId>
        <artifactId>your-artifact-id</artifactId>
        <version>your-version</version>
    </dependency>
</dependencies>

4. 运行目标程序，并加载插件

最后，我们需要运行目标程序，并通过-javaagent命令行参数来启用插件。例如：

java -javaagent:path/to/your/agent.jar your.MainClass

注意，该命令需要在项目的根目录下执行，否则可能会因为路径问题导致插件无法加载。

示例一：

假设我们要为一个Web应用程序添加一个每次请求执行前输出请求IP的功能。那么可以通过以下插件代码实现：

public class RequestInfoAgent {
    public static void premain(String agentArgs, Instrumentation inst) {
        ClassFileTransformer transformer = new RequestInfoTransformer();
        inst.addTransformer(transformer);
    }
}

public class RequestInfoTransformer implements ClassFileTransformer {
    @Override
    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
            ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
        // 只对Web请求相关的类做处理
        if (!className.startsWith("com/example/web")) {
            return null;
        }

        // 使用ASM框架对字节码进行修改
        ClassReader reader = new ClassReader(classfileBuffer);
        ClassWriter writer = new ClassWriter(reader, ClassWriter.COMPUTE_MAXS);
        RequestInfoVisitor visitor = new RequestInfoVisitor(writer);
        reader.accept(visitor, 0);

        return writer.toByteArray();
    }
}

public class RequestInfoVisitor extends ClassVisitor {
    public RequestInfoVisitor(ClassVisitor classVisitor) {
        super(ASM9, classVisitor);
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
        MethodVisitor mv = cv.visitMethod(access, name, descriptor, signature, exceptions);
        if (!name.equals("doGet")) {
            return mv;
        }

        // 添加代码：输出请求IP
        mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
        mv.visitLdcInsn("Request IP: ");
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "print", "(Ljava/lang/String;)V", false);
        mv.visitVarInsn(ALOAD, 1);
        mv.visitMethodInsn(INVOKEINTERFACE, "javax/servlet/http/HttpServletRequest", "getRemoteAddr", "()Ljava/lang/String;", true);
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);

        return mv;
    }
}

生成jar包，并将其配置为Web应用程序的依赖项。然后启动Tomcat服务器，在启动参数中添加-javaagent:path/to/your/agent.jar。访问Web应用程序的URL时，就可以看到每个请求都会输出请求IP。

示例二：

假设我们要在Java应用程序中监控方法的执行时间，那么可以通过以下插件代码实现：

public class MethodTimeAgent {
    public static void premain(String agentArgs, Instrumentation inst) {
        ClassFileTransformer transformer = new MethodTimeTransformer();
        inst.addTransformer(transformer);
    }
}

public class MethodTimeTransformer implements ClassFileTransformer {
    @Override
    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
            ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
        // 只对指定包中的类做处理
        if (!className.startsWith("com/example/app")) {
            return null;
        }

        // 使用ASM框架对字节码进行修改
        ClassReader reader = new ClassReader(classfileBuffer);
        ClassWriter writer = new ClassWriter(reader, ClassWriter.COMPUTE_MAXS);
        MethodTimeVisitor visitor = new MethodTimeVisitor(writer, className);
        reader.accept(visitor, 0);

        return writer.toByteArray();
    }
}

public class MethodTimeVisitor extends ClassVisitor {
    private String className;

    public MethodTimeVisitor(ClassVisitor classVisitor, String className) {
        super(ASM7, classVisitor);
        this.className = className;
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
        MethodVisitor mv = cv.visitMethod(access, name, descriptor, signature, exceptions);
        mv.visitCode();

        // 添加代码：开始计时
        mv.visitMethodInsn(INVOKESTATIC, "java/lang/System", "nanoTime", "()J", false);
        mv.visitFieldInsn(PUTSTATIC, className, "startTime", "J");

        Label l0 = new Label();
        mv.visitLabel(l0);

        mv.visitLineNumber(1, l0);

        // 添加原始方法的代码
        mv = new TimingAdviceAdapter(api, access, name, descriptor, mv, className);

        Label l1 = new Label();
        mv.visitLabel(l1);

        mv.visitLineNumber(1, l1);

        // 添加代码：打印方法执行时间
        mv.visitMethodInsn(INVOKESTATIC, "java/lang/System", "nanoTime", "()J", false);
        mv.visitFieldInsn(GETSTATIC, className, "startTime", "J");
        mv.visitInsn(LSUB);
        mv.visitFieldInsn(PUTSTATIC, className, "methodTime", "J");
        mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
        mv.visitTypeInsn(NEW, "java/lang/StringBuilder");
        mv.visitInsn(DUP);
        mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "()V", false);
        mv.visitLdcInsn("Method executed: ");
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
        mv.visitLdcInsn(name);
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
        mv.visitLdcInsn(", time consumed: ");
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
        mv.visitFieldInsn(GETSTATIC, className, "methodTime", "J");
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(J)Ljava/lang/StringBuilder;", false);
        mv.visitLdcInsn(" ns");
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false);
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);

        mv.visitMaxs(3, 3);
        mv.visitEnd();
        return mv;
    }
}

public class TimingAdviceAdapter extends AdviceAdapter {
    protected TimingAdviceAdapter(int api, int access, String name, String descriptor, MethodVisitor mv, String className) {
        super(api, mv, access, name, descriptor);
        this.className = className;
    }

    @Override
    protected void onMethodExit(int opcode) {
        /*
            方法结束时执行的代码
            这里需要重写onMethodExit，否则在方法退出时会报ClassFormatError
         */

        // 添加代码：结束计时
        mv.visitMethodInsn(INVOKESTATIC, "java/lang/System", "nanoTime", "()J", false);
        mv.visitFieldInsn(PUTSTATIC, className, "endTime", "J");
    }
}

生成jar包，并将其配置为Java应用程序的依赖项。然后启动Java应用程序，在启动参数中添加-javaagent:path/to/your/agent.jar。每次执行包含该插件的方法时，都会输出方法执行时间。

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