Java 中的默认线程

Question

如果代码中没有用户创建的线程，那么当我们调用 jvm 时会启动多少个默认线程？我已经知道将启动 2 个线程，它们是

1）主线程（非守护线程）

2）垃圾收集器（守护线程）

Answer 1

答案取决于您的 JVM。在我的程序中，无操作 Java 程序会产生以下线程：

2011-04-11 10:25:44
Full thread dump Java HotSpot(TM) 64-Bit Server VM (19.1-b02 mixed mode):

"Attach Listener" daemon prio=10 tid=0x0000000040222000 nid=0x606a waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"Low Memory Detector" daemon prio=10 tid=0x0000000040211000 nid=0x6046 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"CompilerThread1" daemon prio=10 tid=0x00007f80d0004000 nid=0x6045 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"CompilerThread0" daemon prio=10 tid=0x00007f80d0001000 nid=0x6044 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"Signal Dispatcher" daemon prio=10 tid=0x000000004020d800 nid=0x6043 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"Finalizer" daemon prio=10 tid=0x00000000401eb000 nid=0x6042 in Object.wait() [0x00007f80d6053000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000000eb561300> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118)
    - locked <0x00000000eb561300> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)

   Locked ownable synchronizers:
    - None

"Reference Handler" daemon prio=10 tid=0x00000000401e9000 nid=0x6041 in Object.wait() [0x00007f80d6154000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000000eb5611d8> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:485)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
    - locked <0x00000000eb5611d8> (a java.lang.ref.Reference$Lock)

   Locked ownable synchronizers:
    - None

"main" prio=10 tid=0x0000000040184800 nid=0x603b runnable [0x00007f80db081000]
   java.lang.Thread.State: RUNNABLE
    at c.main(c.java:1)

   Locked ownable synchronizers:
    - None

"VM Thread" prio=10 tid=0x00000000401e2000 nid=0x6040 runnable 

"GC task thread#0 (ParallelGC)" prio=10 tid=0x0000000040197800 nid=0x603c runnable 

"GC task thread#1 (ParallelGC)" prio=10 tid=0x0000000040199800 nid=0x603d runnable 

"GC task thread#2 (ParallelGC)" prio=10 tid=0x000000004019b800 nid=0x603e runnable 

"GC task thread#3 (ParallelGC)" prio=10 tid=0x000000004019d000 nid=0x603f runnable 

"VM Periodic Task Thread" prio=10 tid=0x000000004021c000 nid=0x6047 waiting on condition 

您可以使用 jstack -l <​​pid> 亲自查看。

Answer 2

如果安装了 JDK，则可以使用位于 bin (Windows) 目录中的 jvisualvm 或 jconsole（旧版本的 Java）。

该工具将允许您查看所有线程及其详细信息。

附截图。