Lambda表达式实现机制的分析

1. 背景

几年前曾经写过一篇Lambda表达式实现机制的分析，现在回头看存在一些错误和疏漏，有误导嫌疑，因此重写一版订正。

平台需求：本文所示代码需要在jdk 15上编译和运行。

2. Lambda表达式的字节码分析

下面代码为一个Lambda表达式的例子

package demo;

import java.util.LinkedList;
import java.util.List;
import java.util.stream.Collectors;

public class Test1 {
   
    public void func1(List<Integer> list) {
   
        if (list == null) {
   
            return;
        }

        var newList = list.stream()
            .map(x -> {
   
                System.out.println("in original lambda " + x);
                return x + x;
            })
            .collect(Collectors.toList());
        var s = """
            oldList = %s
            newList = %s
        """;
        System.out.println(String.format(s, list, newList));
    }

    public static void main(String[] args) {
   
        var t1 = new Test1();

        var list = new LinkedList<Integer>();
        list.add(1);
        list.add(2);

        t1.func1(list);
    }
}

使用javap -c -p -v Test1.class反编译，主要字节码如下所示：

  public void func1(java.util.List<java.lang.Integer>);
    descriptor: (Ljava/util/List;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=6, locals=4, args_size=2
         0: aload_1
         1: ifnonnull     5
         4: return
         5: aload_1
         6: invokeinterface #7,  1            // InterfaceMethod java/util/List.stream:()Ljava/util/stream/Stream;
        11: invokedynamic #13,  0             // InvokeDynamic #0:apply:()Ljava/util/function/Function;
        16: invokeinterface #17,  2           // InterfaceMethod java/util/stream/Stream.map:(Ljava/util/function/Function;)Ljava/util/stream/Stream;
        21: invokestatic  #23                 // Method java/util/stream/Collectors.toList:()Ljava/util/stream/Collector;
        24: invokeinterface #29,  2           // InterfaceMethod java/util/stream/Stream.collect:(Ljava/util/stream/Collector;)Ljava/lang/Object;
        29: checkcast     #8                  // class java/util/List
        32: astore_2
        33: ldc           #33                 // String     oldList = %s\n    newList = %s\n
        35: astore_3
        36: getstatic     #35                 // Field java/lang/System.out:Ljava/io/PrintStream;
        39: aload_3
        40: iconst_2
        41: anewarray     #2                  // class java/lang/Object
        44: dup
        45: iconst_0
        46: aload_1
        47: aastore
        48: dup
        49: iconst_1
        50: aload_2
        51: aastore
        52: invokestatic  #41                 // Method java/lang/String.format:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/String;
        55: invokevirtual #47                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        58: return
      LineNumberTable:
        line 9: 0
        line 10: 4
        line 13: 5
        line 14: 16
        line 18: 21
        line 19: 33
        line 23: 36
        line 24: 58
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      59     0  this   Ldemo/Test1;
            0      59     1  list   Ljava/util/List;
           33      26     2 newList   Ljava/util/List;
           36      23     3     s   Ljava/lang/String;
      LocalVariableTypeTable:
        Start  Length  Slot  Name   Signature
            0      59     1  list   Ljava/util/List<Ljava/lang/Integer;>;
           33      26     2 newList   Ljava/util/List<Ljava/lang/Integer;>;
      StackMapTable: number_of_entries = 1
        frame_type = 5 /* same */
    Signature: #95                          // (Ljava/util/List<Ljava/lang/Integer;>;)V

  private static java.lang.Integer lambda$func1$0(java.lang.Integer);
    descriptor: (Ljava/lang/Integer;)Ljava/lan