你能在运行时检查Java 8 lambda的字节码吗？ [英] Can you inspect the byte code of a Java 8 lambda at runtime?

问题描述

如果您有匿名类，例如

Predicate<String> isEmpty = new Predicate<String>() {
    public boolean test(String t) {
        return t.isEmpty();
    }
};

传递引用的库isEmpty 可以检查字节代码以查看它的作用并可能对其进行操作。有没有办法可以为lambdas做这个？

A library which is passed the reference to isEmpty can inspect the byte code to see what it does and possibly manipulate it. Is there a way you can do this for lambdas?

Predicate<String> isEmpty = String::isEmpty;

例如，假设有这个代码和字节代码

e.g Say have this code and byte code

public class Main {
    public static void test(Predicate<String> tester) {
        System.out.println("tester.getClass()= " + tester.getClass());
        System.out.println("tester.getClass().getClassLoader()="+ tester.getClass().getClassLoader());
    }
    public static void main(String... args) {
        Predicate<String> isEmpty = String::isEmpty;
        test(isEmpty);
    }
}

$ javap -cp . -c -private Main.class
Compiled from "Main.java"
public class Main {
  public Main();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public static void test(java.util.function.Predicate<java.lang.String>);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: new           #3                  // class java/lang/StringBuilder
       6: dup           
       7: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
      10: ldc           #5                  // String tester.getClass()= 
      12: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      15: aload_0       
      16: invokevirtual #7                  // Method java/lang/Object.getClass:()Ljava/lang/Class;
      19: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      22: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      25: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      28: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      31: new           #3                  // class java/lang/StringBuilder
      34: dup           
      35: invokespecial #4                  // Method java/lang/StringBuilder."<init>":()V
      38: ldc           #11                 // String tester.getClass().getClassLoader()=
      40: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      43: aload_0       
      44: invokevirtual #7                  // Method java/lang/Object.getClass:()Ljava/lang/Class;
      47: invokevirtual #12                 // Method java/lang/Class.getClassLoader:()Ljava/lang/ClassLoader;
      50: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      53: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      56: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      59: return        

  public static void main(java.lang.String...);
    Code:
       0: invokedynamic #13,  0             // InvokeDynamic #0:test:()Ljava/util/function/Predicate;
       5: astore_1      
       6: aload_1       
       7: invokestatic  #14                 // Method test:(Ljava/util/function/Predicate;)V
      10: return        
}

在<中提及测试人员 code> test 如何找到调用的方法？

With a reference to tester in test how do I find which method is called?

推荐答案

如果你只是想要查看字节码：

If you just want to SEE the bytecode:

javap -c -p -v classfile
      ^disassemble
         ^private methods
            ^verbose, including constant pool and bootstrap methods attribute

但是如果你想尝试要在运行时这样做，你几乎没有运气（按照设计，我们没有像Expression Trees这样的东西），正如另一个答案所暗示的那样。

But if you want to try to do this at runtime, you're pretty much out of luck (by design, we don't have anything like Expression Trees), as the other answer suggests.

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