湫枫染墨

Lambda parallel 死锁

4/1/2022 Java

摘要

JDK：1.8.0_202

# 一：前言

先看下面两个代码块，他们运行中都是会有问题的。

# 1.1 代码1

public class Client1 {

    private static List<String> list = new ArrayList<>();

    static {
        System.out.println("================start================");
        list.addAll(IntStream.range(0, 3).mapToObj(e -> String.format("第 %s 位", e)).collect(Collectors.toList()));
        System.out.println("=====================================");
        list.addAll(IntStream.range(3, 6).parallel().mapToObj(e -> String.format("第 %s 位", e)).collect(Collectors.toList()));
        System.out.println("================end================");
    }

    public static void main(String[] args) {
        print();
    }

    private static void print() {
        list.forEach(System.out::println);
    }
}
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运行结果

# 1.2 代码2

public class Client2 {

    private static List<String> list = new ArrayList<>();

    static {
        System.out.println("================run================");
        System.out.println("static：" + Thread.currentThread().getName());
        List<String> temp = IntStream.range(0, 10).mapToObj(String::valueOf).collect(Collectors.toList());
        System.out.println("====================================");
        System.out.println("static：" + Thread.currentThread().getName());
        list.addAll(temp.parallelStream().map(e -> String.format("第 %s 位", e)).collect(Collectors.toList()));
        System.out.println("================end================");
    }

    public static void main(String[] args) {
        System.out.println("main：" + Thread.currentThread().getName());
        print();
    }

    private static void print() {
        list.forEach(System.out::println);
    }
}
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运行结果

# 1.3 有何问题

可以发现项目运行到 parallel 相关的代码的时候，就会 "卡住"，同时如果看堆栈信息，就会发现有报错。

# 二：问题原因

下面以上面代码中，第二个例子结果进行说明，先看结果打印的最上层堆栈信息

如果觉得不够详细，可以查看下面更加详细的堆栈信息

"ForkJoinPool.commonPool-worker-1" #20 daemon prio=5 os_prio=0 tid=0x000000001f815000 nid=0x17260 in Object.wait() [0x000000002096e000]
   java.lang.Thread.State: RUNNABLE
	at com.ccjjltx.temp.Client2$$Lambda$6/1607521710.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
	at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"ForkJoinPool.commonPool-worker-6" #17 daemon prio=5 os_prio=0 tid=0x000000001f81c000 nid=0x18798 in Object.wait() [0x000000002086e000]
   java.lang.Thread.State: RUNNABLE
	at com.ccjjltx.temp.Client2$$Lambda$6/1607521710.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
	at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"ForkJoinPool.commonPool-worker-8" #19 daemon prio=5 os_prio=0 tid=0x000000001f819800 nid=0x16adc in Object.wait() [0x000000002076e000]
   java.lang.Thread.State: RUNNABLE
	at com.ccjjltx.temp.Client2$$Lambda$6/1607521710.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
	at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"ForkJoinPool.commonPool-worker-15" #18 daemon prio=5 os_prio=0 tid=0x000000001f816800 nid=0x17138 in Object.wait() [0x000000002066e000]
   java.lang.Thread.State: RUNNABLE
	at com.ccjjltx.temp.Client2$$Lambda$6/1607521710.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
	at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"ForkJoinPool.commonPool-worker-13" #16 daemon prio=5 os_prio=0 tid=0x000000001f814800 nid=0x14e68 in Object.wait() [0x000000002046e000]
   java.lang.Thread.State: RUNNABLE
	at com.ccjjltx.temp.Client2$$Lambda$6/1607521710.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
	at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"ForkJoinPool.commonPool-worker-4" #15 daemon prio=5 os_prio=0 tid=0x000000001f808800 nid=0x15570 in Object.wait() [0x000000002036e000]
   java.lang.Thread.State: RUNNABLE
	at com.ccjjltx.temp.Client2$$Lambda$6/1607521710.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
	at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"ForkJoinPool.commonPool-worker-11" #14 daemon prio=5 os_prio=0 tid=0x000000001f7fc000 nid=0x1704 in Object.wait() [0x000000002026e000]
   java.lang.Thread.State: RUNNABLE
	at com.ccjjltx.temp.Client2$$Lambda$6/1607521710.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
	at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"ForkJoinPool.commonPool-worker-2" #13 daemon prio=5 os_prio=0 tid=0x000000001f7f6000 nid=0x17a28 in Object.wait() [0x000000002016e000]
   java.lang.Thread.State: RUNNABLE
	at com.ccjjltx.temp.Client2$$Lambda$6/1607521710.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
	at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"ForkJoinPool.commonPool-worker-9" #12 daemon prio=5 os_prio=0 tid=0x000000001f7f5800 nid=0xd10c in Object.wait() [0x000000002006e000]
   java.lang.Thread.State: RUNNABLE
	at com.ccjjltx.temp.Client2$$Lambda$6/1607521710.apply(Unknown Source)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
	at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
	at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"main" #1 prio=5 os_prio=0 tid=0x0000000000d3e800 nid=0x13374 in Object.wait() [0x0000000002a9e000]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x000000076f3028e0> (a java.util.stream.ReduceOps$ReduceTask)
	at java.util.concurrent.ForkJoinTask.externalAwaitDone(ForkJoinTask.java:334)
	- locked <0x000000076f3028e0> (a java.util.stream.ReduceOps$ReduceTask)
	at java.util.concurrent.ForkJoinTask.doInvoke(ForkJoinTask.java:405)
	at java.util.concurrent.ForkJoinTask.invoke(ForkJoinTask.java:734)
	at java.util.stream.ReduceOps$ReduceOp.evaluateParallel(ReduceOps.java:714)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:233)
	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
	at com.ccjjltx.temp.Client2.<clinit>(Client2.java:18)


"ForkJoinPool.commonPool-worker-10" #21 daemon prio=5 os_prio=0 tid=0x000000001f81a800 nid=0x1616c waiting on condition [0x0000000020a6f000]
   java.lang.Thread.State: WAITING (parking)
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x000000076f302740> (a java.util.concurrent.ForkJoinPool)
	at java.util.concurrent.ForkJoinPool.awaitWork(ForkJoinPool.java:1824)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1693)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x000000001db86800 nid=0x11dc0 runnable [0x000000001f16e000]
   java.lang.Thread.State: RUNNABLE
	at java.net.SocketInputStream.socketRead0(Native Method)
	at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
	at java.net.SocketInputStream.read(SocketInputStream.java:171)
	at java.net.SocketInputStream.read(SocketInputStream.java:141)
	at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
	at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
	at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
	- locked <0x000000076ee78960> (a java.io.InputStreamReader)
	at java.io.InputStreamReader.read(InputStreamReader.java:184)
	at java.io.BufferedReader.fill(BufferedReader.java:161)
	at java.io.BufferedReader.readLine(BufferedReader.java:324)
	- locked <0x000000076ee78960> (a java.io.InputStreamReader)
	at java.io.BufferedReader.readLine(BufferedReader.java:389)
	at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:49)


"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x000000001c3e3000 nid=0x183c4 in Object.wait() [0x000000001ee6f000]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x000000076ed08ed0> (a java.lang.ref.ReferenceQueue$Lock)
	at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
	- locked <0x000000076ed08ed0> (a java.lang.ref.ReferenceQueue$Lock)
	at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)
	at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:216)


"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x000000001dad3000 nid=0x11430 in Object.wait() [0x000000001ed6f000]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x000000076ed06bf8> (a java.lang.ref.Reference$Lock)
	at java.lang.Object.wait(Object.java:502)
	at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
	- locked <0x000000076ed06bf8> (a java.lang.ref.Reference$Lock)
	at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)


"Service Thread" #11 daemon prio=9 os_prio=0 tid=0x000000001dc45800 nid=0x17848 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE


"C1 CompilerThread3" #10 daemon prio=9 os_prio=2 tid=0x000000001db95000 nid=0x17628 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE


"C2 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x000000001db93000 nid=0x14380 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE


"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x000000001db92800 nid=0x15de0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE


"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x000000001db89000 nid=0x17bbc waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE


"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x000000001dae8000 nid=0x160cc waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE


"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x000000001db4a800 nid=0x1954 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE


"VM Thread" os_prio=2 tid=0x000000001c3d5800 nid=0x13430 runnable 


"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000002d18800 nid=0x18848 runnable 


"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x0000000002d1a000 nid=0x14cf0 runnable 


"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x0000000002d1b800 nid=0x15228 runnable 


"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x0000000002d1d000 nid=0xb954 runnable 


"GC task thread#4 (ParallelGC)" os_prio=0 tid=0x0000000002d20800 nid=0x148dc runnable 


"GC task thread#5 (ParallelGC)" os_prio=0 tid=0x0000000002d21800 nid=0xca10 runnable 


"GC task thread#6 (ParallelGC)" os_prio=0 tid=0x0000000002d24800 nid=0x13220 runnable 


"GC task thread#7 (ParallelGC)" os_prio=0 tid=0x0000000002d26000 nid=0x14f30 runnable 


"GC task thread#8 (ParallelGC)" os_prio=0 tid=0x0000000002d27000 nid=0xd5b0 runnable 


"GC task thread#9 (ParallelGC)" os_prio=0 tid=0x0000000002d28000 nid=0x151b8 runnable 


"VM Periodic Task Thread" os_prio=2 tid=0x000000001dc73000 nid=0x177c0 waiting on condition 
JNI global references: 328
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可以发现 ForkJoinPool.commonPool-worke-* 的线程是 RUNNABLE，这个状态并不一定是正在执行，也可能是准备好了，等待执行。

1. main线程等待ForkJoinPool.commonPool-worker-*线程

ForkJoinPool.commonPool-worker-*执行完，外部类的静态代码块才能执行完，main线程才能往下走

2. ForkJoinPool.commonPool-worker-*线程等待main线程

因为ForkJoinPool要执行lamda，说白了就是这个类的一个内部类，这个类正在有main线程完成初始化。

这样就是两个线程互相等待，造成死锁了

对于这个问题，之前就有人给jdk官方提过bug： https://bugs.openjdk.java.net/browse/JDK-8143380 (opens new window) 但是jdk官方认为这个不是bug，是使用的人自己的问题，就关闭了这个问题

同理，下面这种写法一样不可行：

private static final Map<String, String> STRUCT_MAP = Arrays.asList("a", "b", "c").
		parallelStream().filter(e -> e != null).collect(Collectors.toMap(e -> e, e -> e, (o1, o2) -> o1));
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# 三：解决方法

方法一：

静态代码块不使用并发流（parallel()），使用普通的流就好了

方法二：

分一个子线程执行 static

public class Client3 {

    private static List<String> list = new ArrayList<>();

    static {
        new Thread(() -> {
            System.out.println("================run================");
            System.out.println("static：" + Thread.currentThread().getName());
            List<String> temp = IntStream.range(0, 10).mapToObj(String::valueOf).collect(Collectors.toList());
            System.out.println("====================================");
            System.out.println("static：" + Thread.currentThread().getName());
            list.addAll(temp.parallelStream().map(e -> String.format("第 %s 位", e)).collect(Collectors.toList()));
            System.out.println("================end================");
        }).start();
    }

    public static void main(String[] args) {
        System.out.println("main：" + Thread.currentThread().getName());
        print();
    }

    private static void print() {
        list.forEach(System.out::println);
    }

}
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运行结果：

# 四：参考文献

最后更新: 4/1/2022, 1:49:22 PM

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