Java编译器是否优化流过滤? [英] Does Java compiler optimize stream filtering?

问题描述

让我们来一个案例:

x.stream().filter(X::isFlag).filter(this::isOtherFlag).reduce(...)

与这个不同吗?

x.stream().filter(predicate(X::isFlag).and(this::isOtherFlag)).reduce(...)

推荐答案

从功能上讲，这两个语句是等效的.但是，请考虑以下两个代码块及其各自的字节码:

Functionally, the two statements are equivalent. However, consider the two following blocks of code and their respective bytecodes:

public static void main(String[] args) {
    List<String> list = List.of("Seven", "Eight", "Nine");

    list.stream().filter(s -> s.length() >= 5)
                 .filter(s -> s.contains("n"))
                 .forEach(System.out::println);
}

public static void main(java.lang.String[]);
Code:
   0: ldc           #16                 // String Seven
   2: ldc           #18                 // String Eight
   4: ldc           #20                 // String Nine
   6: invokestatic  #22                 // InterfaceMethod java/util/List.of:(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/util/List;
   9: astore_1
  10: aload_1
  11: invokeinterface #28,  1           // InterfaceMethod java/util/List.stream:()Ljava/util/stream/Stream;
  16: invokedynamic #35,  0             // InvokeDynamic #0:test:()Ljava/util/function/Predicate;
  21: invokeinterface #36,  2           // InterfaceMethod java/util/stream/Stream.filter:(Ljava/util/function/Predicate;)Ljava/util/stream/Stream;
  26: invokedynamic #42,  0             // InvokeDynamic #1:test:()Ljava/util/function/Predicate;
  31: invokeinterface #36,  2           // InterfaceMethod java/util/stream/Stream.filter:(Ljava/util/function/Predicate;)Ljava/util/stream/Stream;
  36: getstatic     #43                 // Field java/lang/System.out:Ljava/io/PrintStream;
  39: invokedynamic #52,  0             // InvokeDynamic #2:accept:(Ljava/io/PrintStream;)Ljava/util/function/Consumer;
  44: invokeinterface #53,  2           // InterfaceMethod java/util/stream/Stream.forEach:(Ljava/util/function/Consumer;)V
  49: return

-

public static void main(String[] args) {
    List<String> list = List.of("Seven", "Eight", "Nine");

    list.stream().filter(s -> s.length() >= 5 && s.contains("n"))
                 .forEach(System.out::println);
}

public static void main(java.lang.String[]);
Code:
   0: ldc           #16                 // String Seven
   2: ldc           #18                 // String Eight
   4: ldc           #20                 // String Nine
   6: invokestatic  #22                 // InterfaceMethod java/util/List.of:(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/util/List;
   9: astore_1
  10: aload_1
  11: invokeinterface #28,  1           // InterfaceMethod java/util/List.stream:()Ljava/util/stream/Stream;
  16: invokedynamic #35,  0             // InvokeDynamic #0:test:()Ljava/util/function/Predicate;
  21: invokeinterface #36,  2           // InterfaceMethod java/util/stream/Stream.filter:(Ljava/util/function/Predicate;)Ljava/util/stream/Stream;
  26: getstatic     #42                 // Field java/lang/System.out:Ljava/io/PrintStream;
  29: invokedynamic #51,  0             // InvokeDynamic #1:accept:(Ljava/io/PrintStream;)Ljava/util/function/Consumer;
  34: invokeinterface #52,  2           // InterfaceMethod java/util/stream/Stream.forEach:(Ljava/util/function/Consumer;)V
  39: return

我们可以看到，在第二个示例中，缺少对invokedynamic和invokeinterface的一个调用(这很有意义，因为我们省略了对filter的调用).我确定有人可以协助我对该字节码进行静态分析(如果需要，我可以发布详细文件)，但是Java编译器显然将对filter的单个调用视为单个Predicate<String>，而不是将其拆分为&&运算符，将字节码略微缩短.

We can see that, in the second example, one call to invokedynamic and invokeinterface are missing (which makes sense as we omitted a call to filter). I'm sure someone could assist with me with the static analysis of this bytecode (I can post verbose files if needed), but the Java compiler clearly treats the single call to filter as a single Predicate<String> rather than splitting it at the operator &&, shortening the bytecode slightly.

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