stream中forEach和forEachOrdered使用和区别

Posted 2022-02-14 泡^泡

测试例子

package com;
import java.util.stream.Stream;

public class App 

    public static void main( String[] args )
    
        Stream.of("张三,","李四,","王五,","马六,").parallel().forEach(System.out::print);
        System.out.println("\\n______________________________________________");
        Stream.of("张三,","李四,","王五,","马六,").parallel().forEachOrdered(System.out::print);
        System.out.println("\\n______________________________________________");
        Stream.of("张三,","李四,","王五,","马六,").parallel().forEachOrdered(System.out::print);
    

首先对forEach来说,当stream为parallel的时候,它是并行处理多线程的，所以不能保证输出数据的顺序，但是处理数据的效率比较高。操作共享变量的时候,需要考虑线程安全的问题。

错误案例

package com;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class App 

    public static void main( String[] args )
    
        List<Integer> list = new ArrayList<>();
        IntStream.range(0,10000).parallel().forEach(item->list.add(item));

代码修正

package com;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class App 

    public static void main( String[] args )
    
        List<Integer> list = new ArrayList<>();
        IntStream.range(0,10000).parallel().forEachOrdered(item->list.add(item));

对于使用forEachOrdeed ，当stream 为parallel的时候，虽然是多个线程并行处理的。但是还是会按照他source原有的顺序输出的，底层是通过happensbefore原则保证了它的内存可见性。