摘抄的 Java 字节码编程知识

字节码编程

方法一：修改类文件

首先定义 Item 类，位于 set 包内

package set;

import bytecodeAnnotations.LogEntry;

import java.util.Objects;

/**
 * An item with a description and a part number.
 *
 * @author qusong
 * @Date 2022/11/1
 **/
public class Item {
    private String description;
    private int partNumber;

    /**
     * Constructs an item.
     *
     * @param description the item's description
     * @param partNumber  the item's part number
     */
    public Item(String description, int partNumber) {
        this.description = description;
        this.partNumber = partNumber;
    }

    /**
     * Gets the description of this item.
     *
     * @return the description
     */
    public String getDescription() {
        return description;
    }

    @Override
    public String toString() {
        return "[description=" + description +
                ", partNumber=" + partNumber + "]";
    }

    @LogEntry(logger = "com.qusong")
    @Override
    public boolean equals(Object otherObject) {
        if (this == otherObject) return true;
        if (otherObject == null) return false;
        if (getClass() != otherObject.getClass()) return false;
        Item other = (Item) otherObject;
        return Objects.equals(description, other.description)
                && partNumber == other.partNumber;
    }

    @LogEntry(logger = "com.qusong")
    @Override
    public int hashCode() {
        return Objects.hash(description, partNumber);
    }
}

其中的 equals 方法和 hashcode 方法前添加了自定义的注解 @LogEntry(logger = "com.qusong") ，该注解的定义如下：

package bytecodeAnnotations;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface LogEntry {
    String logger();
}

再定义EntryLogger类用来解析 LogEntry 注解，实现为所有带LogEntry注解的方法在方法起始处添加log语句的功能。

EntryLogger 类的main 方法需要一个参数，该参数指定了要向哪个类文件中插入字节码。

本文中是要向 set/Item.class 文件中插入字节码，因此命令行参数需添加 item.class 文件的路径信息：java bytecodeAnnotations.EntryLogger set/Item.class

package bytecodeAnnotations;

import jdk.internal.org.objectweb.asm.*;
import jdk.internal.org.objectweb.asm.commons.AdviceAdapter;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

/**
 * Adds "entering" logs to all methods of a class
 * that have the LogEntry annotation.
 *
 * @author qusong
 * @Date 2022/11/1
 **/
public class EntryLogger extends ClassVisitor {
    private String className;

    /**
     * Constructs an EntryLogger that inserts logging into
     * annotated methods of a given class.
     *
     * @param writer
     * @param className
     */
    public EntryLogger(ClassWriter writer, String className) {
        super(Opcodes.ASM5, writer);
        this.className = className;
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String desc,
                                     String signature, String[] exceptions) {
        MethodVisitor mv = cv.visitMethod(access, name, desc, signature, exceptions);
        return new AdviceAdapter(Opcodes.ASM5, mv, access, name, desc) {
            private String loggerName;

            @Override
            public AnnotationVisitor visitAnnotation(String desc, boolean visible) {
                return new AnnotationVisitor(Opcodes.ASM5) {
                    @Override
                    public void visit(String name, Object value) {
                        if (desc.equals("LbytecodeAnnotations/LogEntry;")
                                && name.equals("logger"))
                            loggerName = value.toString();
                    }
                };
            }

            @Override
            protected void onMethodEnter() {
                if (loggerName != null) {
                    visitLdcInsn(loggerName);
                    visitMethodInsn(INVOKESTATIC, "java/util/logging/Logger",
                            "getLogger", "(Ljava/lang/String;)Ljava/util/logging/Logger;", false);
                    visitLdcInsn(className);
                    visitLdcInsn(name);
                    visitMethodInsn(INVOKEVIRTUAL, "java/util/logging/Logger",
                            "entering", "(Ljava/lang/String;Ljava/lang/String;)V", false);
                    loggerName = null;
                }
            }
        };
    }

    /**
     * Adds entry logging code to the given class.
     *
     * @param args the name of the class file to path
     */
    public static void main(String[] args) throws IOException {
        if (args.length == 0) {
            System.out.println("USAGE: java bytecodeAnnotations.EntryLogger classfile");
            System.exit(1);
        }
        Path path = Paths.get(args[0]);
        ClassReader reader = new ClassReader(Files.newInputStream(path));
        ClassWriter writer = new ClassWriter(
                ClassWriter.COMPUTE_MAXS | ClassWriter.COMPUTE_FRAMES);
        EntryLogger entryLogger = new EntryLogger(writer,
                path.toString().replace(".class", "").
                        replaceAll("[/\\\\]", "."));
        reader.accept(entryLogger, ClassReader.EXPAND_FRAMES);
        Files.write(Paths.get(args[0]), writer.toByteArray());
    }
}

可以使用 javap -c set.Item 查看插入字节码前后的指令区别

下面只粘贴了 Item 类中 hashCode 方法相关的字节码信息：

插入前

public int hashCode();
   Code:
      0: iconst_2
      1: anewarray     #15                 // class java/lang/Object
      4: dup
      5: iconst_0
      6: aload_0
      7: getfield      #2                  // Field description:Ljava/lang/String;
     10: aastore
     11: dup
     12: iconst_1
     13: aload_0
     14: getfield      #3                  // Field partNumber:I
     17: invokestatic  #16                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
     20: aastore
     21: invokestatic  #17                 // Method java/util/Objects.hash:([Ljava/lang/Object;)I
     24: ireturn

插入后

public int hashCode();
    Code:
       0: ldc           #45                 // String com.qusong
       2: invokestatic  #51                 // Method java/util/logging/Logger.getLogger:(Ljava/lang/String;)Ljava/util/logging/Logger;
       5: ldc           #53                 // String .Users.qusong.IdeaProjects.JavaLearn.target.classes.set.Item
       7: ldc           #73                 // String hashCode
       9: invokevirtual #58                 // Method java/util/logging/Logger.entering:(Ljava/lang/String;Ljava/lang/String;)V
      12: iconst_2
      13: anewarray     #4                  // class java/lang/Object
      16: dup
      17: iconst_0
      18: aload_0
      19: getfield      #16                 // Field description:Ljava/lang/String;
      22: aastore
      23: dup
      24: iconst_1
      25: aload_0
      26: getfield      #18                 // Field partNumber:I
      29: invokestatic  #79                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      32: aastore
      33: invokestatic  #83                 // Method java/util/Objects.hash:([Ljava/lang/Object;)I
      36: ireturn

最后，可以在 set 包内定义 SetTest 类来使用 Item 类

package set;

import java.util.HashSet;
import java.util.Set;
import java.util.logging.ConsoleHandler;
import java.util.logging.Handler;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * @author qusong
 * @Date 2022/11/1
 **/
public class SetTest {
    public static void main(String[] args) {
        Logger.getLogger("com.qusong").setLevel(Level.FINEST);
        Handler handler = new ConsoleHandler();
        handler.setLevel(Level.FINEST);
        Logger.getLogger("com.qusong").addHandler(handler);

        Set<Item> parts = new HashSet<>();
        parts.add(new Item("Toaster", 1279));
        parts.add(new Item("Microwave", 4104));
        parts.add(new Item("Toaster", 1279));
        System.out.println(parts);
    }
}

可以看到，调用 parts.add() 方法时，内部调用了 Item 方法的 hashcode 方法，因此会有自动添加的log语句

十一月 01, 2022 2:18:56 下午 .Users.qusong.IdeaProjects.JavaLearn.target.classes.set.Item hashCode
较详细: ENTRY
十一月 01, 2022 2:18:56 下午 .Users.qusong.IdeaProjects.JavaLearn.target.classes.set.Item hashCode
较详细: ENTRY
十一月 01, 2022 2:18:56 下午 .Users.qusong.IdeaProjects.JavaLearn.target.classes.set.Item hashCode
较详细: ENTRY
十一月 01, 2022 2:18:56 下午 .Users.qusong.IdeaProjects.JavaLearn.target.classes.set.Item equals
较详细: ENTRY
[[description=Microwave, partNumber=4104], [description=Toaster, partNumber=1279]]

Process finished with exit code 0

第二次调用parts.add(new Item("Toaster", 1279)) 时，触发了 Item 的 equals 方法，最终parts 内只有两个元素。

方法二：在运行时修改字节

instrumentation API 有一个 hook 可以安装一个字节码变换器，但是这个变换器必须在 main 方法调用前安装，所以需要定义一个 agent（一个用来监控程序的库）来实现。

构建 agent 的步骤如下：

1. 实现一个类，包含 premain 方法

   public static void premain(String arg, Instrumentation instr)

   该方法将会在 agent 被加载时被调用，agent 可以获取一个单行的命令行参数，传入 arg ，另一个参数 instr 则是用来指示不同的 hooks

2. 新建一个 manifest 文件 EntryLoggingAgent.mf 来设置 Premain-Class 的属性，例如

   remain-Class: bytecodeAnnotations.EntryLoggingAgent

3. 将 agent 代码和 manifest 文件打包进 jar 包内

   javac -classpath .:asm/lib/\* bytecodeAnnotations/EntryLoggingAgent.java

   jar cvfm EntryLoggingAgent.jar bytecodeAnnotations/EntryLoggingAgent.mf bytecodeAnnotations/Entry*.class

运行带 agent 的 Java 代码的方法：

java -javaagent:AgentJARFile=agentArgumen

于是，定义 EntryLoggingAgent 类构建 agent

package bytecodeAnnotations;

import jdk.internal.org.objectweb.asm.ClassReader;
import jdk.internal.org.objectweb.asm.ClassWriter;

import java.lang.instrument.Instrumentation;

/**
 * @author qusong
 * @Date 2022/11/1
 **/
public class EntryLoggingAgent {
    public static void premain(final String arg, Instrumentation instr) {
        instr.addTransformer(((loader, className, classBeingRedefined, protectionDomain, data) -> {
            if (!className.equals(arg)) return null;
            ClassReader reader = new ClassReader(data);
            ClassWriter writer = new ClassWriter(
                    ClassWriter.COMPUTE_MAXS|ClassWriter.COMPUTE_FRAMES
            );
            EntryLogger el = new EntryLogger(writer, className);
            reader.accept(el, ClassReader.EXPAND_FRAMES);
            return writer.toByteArray();
        }));
    }
}

使用 javac -classpath .:asm/lib/\* bytecodeAnnotations/EntryLoggingAgent.java 编译EntryLoggingAgent

之后使用 jar 命令打包

jar cvfm EntryLoggingAgent.jar bytecodeAnnotations/EntryLoggingAgent.mf bytecodeAnnotations/Entry*.class

最后使用 java -javaagent:EntryLoggingAgent.jar=set/Item set/SetTest 命令运行，即可得到和方法一同样的输出

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