並行使用BufferedReader.lines（）會中斷

Question

我正在編寫一些代碼，這些代碼將讀取日志行並在后台對該數據進行一些處理。 這種處理可能會受益於並行化，例如Stream.parallel方法提供的內容，而我正在嘗試使用它。 這是我一開始就可以完美運行的代碼。

public static void main(String[] args) {
    try {
        final Socket socket = new Socket(ADDRESS, PORT);
        final BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        socket.getOutputStream().write(QUERY);
        reader.lines().forEach(System.out::println);
    } catch (IOException e) {
        e.printStackTrace();
    }

}

此代碼連接並打印出我所有的數據。 我非常想將這段代碼重組如下：

public static void main(String[] args) {
    try (Socket socket = new Socket(ADDRESS, PORT); 
         BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()))) {
        socket.getOutputStream().write(QUERY);
        reader.lines().forEach(System.out::println);
    } catch (IOException e) {
        e.printStackTrace();
    }

}

但可惜的是這行不通。 更糟糕的是，回到原始代碼，這甚至行不通：

public static void main(String[] args) {
    try {
        final Socket socket = new Socket(ADDRESS, PORT);
        final BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        socket.getOutputStream().write(QUERY);
        reader.lines().parallel().forEach(System.out::println);
    } catch (IOException e) {
        e.printStackTrace();
    }

}

在這里添加的只是一個.parallel調用，這完全不起作用。 它只是坐在那里，什么也沒打印出來。

使用修改后的try(A a = new A()) {} ，即使沒有第二版，我也可以過得很好，因為在這種情況下，看起來不太好。 我不能沒有的生活就是弄清楚為什么這個.parallel調用會破壞一切。

我假設修改后的try語句在流掉后立即關閉流（在啟動forEach之后），因此在運行之前將它們殺死並進行GC處理。 我無法為自己的一生弄清楚.parallel調用到底是怎么回事。

這里要求的是在此代碼的.parellel（）版本上運行的jstack的輸出。

Full thread dump OpenJDK 64-Bit Server VM (25.112-b15 mixed mode):

"Attach Listener" #9 daemon prio=9 os_prio=0 tid=0x00007fd4f4001000 nid=0x4907 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Service Thread" #8 daemon prio=9 os_prio=0 tid=0x00007fd5280be000 nid=0x48d2 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread2" #7 daemon prio=9 os_prio=0 tid=0x00007fd5280bb000 nid=0x48d1 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #6 daemon prio=9 os_prio=0 tid=0x00007fd5280b9800 nid=0x48d0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #5 daemon prio=9 os_prio=0 tid=0x00007fd5280b6800 nid=0x48cf waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=0 tid=0x00007fd5280b5000 nid=0x48ce runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007fd528082000 nid=0x48cd in Object.wait() [0x00007fd515c6d000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000000ec008e98> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
    - locked <0x00000000ec008e98> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

"Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007fd52807d800 nid=0x48cc in Object.wait() [0x00007fd515d6e000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000000ec006b40> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:502)
    at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
    - locked <0x00000000ec006b40> (a java.lang.ref.Reference$Lock)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"main" #1 prio=5 os_prio=0 tid=0x00007fd528008000 nid=0x48c2 runnable [0x00007fd52fd9f000]
   java.lang.Thread.State: RUNNABLE
    at java.net.PlainSocketImpl.socketConnect(Native Method)
    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
    - locked <0x00000000ec086790> (a java.net.SocksSocketImpl)
    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
    at java.net.Socket.connect(Socket.java:589)
    at java.net.Socket.connect(Socket.java:538)
    at java.net.Socket.<init>(Socket.java:434)
    at java.net.Socket.<init>(Socket.java:211)
    at com.gravypod.Test.main(Test.java:48)

"VM Thread" os_prio=0 tid=0x00007fd528075800 nid=0x48ca runnable 

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00007fd52801d800 nid=0x48c4 runnable 

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00007fd52801f000 nid=0x48c5 runnable 

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00007fd528021000 nid=0x48c6 runnable 

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00007fd528022800 nid=0x48c7 runnable 

"VM Periodic Task Thread" os_prio=0 tid=0x00007fd5280c0800 nid=0x48d3 waiting on condition 

JNI global references: 18

Test.java:48行是Socket socket = new Socket行。 這是完全正常工作的非並行代碼（僅使用.lines（））的結果。

Full thread dump OpenJDK 64-Bit Server VM (25.112-b15 mixed mode):

"Attach Listener" #9 daemon prio=9 os_prio=0 tid=0x00007f9048001000 nid=0x4982 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Service Thread" #8 daemon prio=9 os_prio=0 tid=0x00007f90800be800 nid=0x496f runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread2" #7 daemon prio=9 os_prio=0 tid=0x00007f90800bb000 nid=0x496e waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #6 daemon prio=9 os_prio=0 tid=0x00007f90800b9800 nid=0x496d waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #5 daemon prio=9 os_prio=0 tid=0x00007f90800b6800 nid=0x496c waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=0 tid=0x00007f90800b5000 nid=0x496b runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007f9080082000 nid=0x496a in Object.wait() [0x00007f907018d000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000000ec008e98> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
    - locked <0x00000000ec008e98> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

"Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007f908007d800 nid=0x4969 in Object.wait() [0x00007f907028e000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000000ec006b40> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:502)
    at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
    - locked <0x00000000ec006b40> (a java.lang.ref.Reference$Lock)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"main" #1 prio=5 os_prio=0 tid=0x00007f9080008000 nid=0x4961 runnable [0x00007f90884c3000]
   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:170)
    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 <0x00000000ec08e890> (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 <0x00000000ec08e890> (a java.io.InputStreamReader)
    at java.io.BufferedReader.readLine(BufferedReader.java:389)
    at java.io.BufferedReader$1.hasNext(BufferedReader.java:571)
    at java.util.Iterator.forEachRemaining(Iterator.java:115)
    at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
    at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580)
    at com.gravypod.Test.main(Test.java:51)

"VM Thread" os_prio=0 tid=0x00007f9080075800 nid=0x4968 runnable 

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00007f908001d800 nid=0x4963 runnable 

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00007f908001f000 nid=0x4964 runnable 

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00007f9080021000 nid=0x4965 runnable 

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00007f9080022800 nid=0x4966 runnable 

"VM Periodic Task Thread" os_prio=0 tid=0x00007f90800c1000 nid=0x4970 waiting on condition 

JNI global references: 319

Test.java:51行是reader.lines().forEach行。

Answer 1

我想在並行執行任務之前，並行流上的parallel（）或forEach（）等待讀取所有輸入。 因為服務器從不關閉連接，所以它將永遠等待。

您的任務不是真正可並行化的。 數據按順序通過網絡傳輸，因此無法並行讀取。

Answer 2

看來，從技術上講，您的應用程序並未掛起，而只是在執行可觀察的工作之前等待大量輸入。 這是兩個實現細節的組合。 當您開始並行流操作時，它將首先嘗試拆分工作負載，直到每個CPU內核都有所要做的事情為止，然后才真正開始處理元素。 由於不可配置的批處理大小問題，它與Reader＃lines（）的結合嚴重並行化 。

簡而言之，當Stream的大小未知時，實現將嘗試緩沖成批分割的1024倍的大小。 這個好答案表明，對於具有多個核心的未知大小的流，拆分將如何發生，這表明1024元素的倍數將在該過程中得到緩沖。 在調用傳遞給forEach的使用者之前，這可能需要很長時間。

請注意，無論如何，通過非短路forEach處理無限源forEach在Stream API的范圍內。 假設及時的副作用是有關Stream的處理順序的假設，但是對此沒有任何保證。

該答案將指導您解決問題。 您可以使用類似

try(Socket socket = new Socket(ADDRESS, PORT);
    BufferedReader reader = new BufferedReader(
        new InputStreamReader(socket.getInputStream()))) {

    socket.getOutputStream().write(QUERY);
    Stream.generate(() -> {
        try { return reader.readLine(); }
        catch (IOException ex) { throw new UncheckedIOException(ex); }
    }).parallel().forEach(System.out::println);
} catch(IOException|UncheckedIOException e) {
    e.printStackTrace();
}

但是，如上所述，這不是Stream API的預期用例……