Java编译器是否优化流过滤？

Question

举个例子：

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(...)

Answer 1

在功能上，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：

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，缺少一个对invokedynamic和 的调用invokeinterface（这是有道理的，因为我们省略了对 的调用filter）。我敢肯定有人可以帮助我这个字节码的静态分析（我可以张贴详细的文件如果需要的话），但Java编译器清楚地对待单个呼叫filter作为一个单一的Predicate<String>，而不是分裂它在操作&&，缩短了字节码轻微地。