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在Java中有两类线程：

用户线程 (User Thread) 和 守护线程 (Daemon Thread)。

守护线程

1. 是指在程序运行的时候在后台提供一种通用服务的线程,并不属于程序中不可或缺的部分

2. 当所有的非守护线程结束时，程序也就终止了，同时会杀死进程中的所有守护线程

用户线程和守护线程区别：

唯一的不同之处就在于虚拟机的离开：如果用户线程已经全部退出运行了，只剩下守护线程存在了，虚拟机也就退出了。 因为没有了被守护者，守护线程也就没有工作可做了，也就没有继续运行程序的必要了。

将线程转换为守护线程

通过调用Thread对象的setDaemon(true)方法来实现。在使用守护线程时需要注意一下几点：

1. 必须在start()方法之前设置

   thread.setDaemon(true)必须在thread.start()之前设置，否则会跑出一个IllegalThreadStateException异常。你不能把正在运行的常规线程设置为守护线程。

2. 在Daemon线程中产生的新线程也是Daemon的

3. 守护线程应该永远不去访问固有资源，如文件、数据库，因为它会在任何时候甚至在一个操作的中间发生中断

编写实例

1. 编写DeamonThread类实现往文件中写入数据的功能
2. 在主线程中将DeamonThread设置为守护线程 并且使用键盘输入模拟阻塞状态
3. 当从键盘输入后从阻塞状态进入运行状态 主线程执行结束 守护线程也结束

DeamonThread :

package thread.demo3;import java.io.*;/** * 守护线程DeamonThread * Created by heqianqian on 2017/4/15. */public class DeamonThread implements Runnable {
       private int count = 0;    @Override    public void run() {        System.out.println("Deamon Thread Running....");        File file = new File("Base/file/deamon/file.txt");        OutputStream outputStream = null;        try {            outputStream = new FileOutputStream(file, true);            while (count < 999) {                outputStream.write(("word:"+count+"\r\n").getBytes());                System.out.println(Thread.currentThread().getName()+" write word "+count++);                Thread.sleep(1000);            }        } catch (FileNotFoundException e) {            e.printStackTrace();        } catch (IOException e) {            e.printStackTrace();        } catch (InterruptedException e) {            e.printStackTrace();        } finally {            if (outputStream!=null){                try {                    outputStream.close();                } catch (IOException e) {                    e.printStackTrace();                }            }        }        System.out.println("Deamon Thread Over!");    }}

Main:

package thread.demo3;import java.util.Scanner;/** * Main * Created by heqianqian on 2017/4/15. */public class Main {
       public static void main(String[] args) {        System.out.println("Main Thread Running....");        DeamonThread deamonThread = new DeamonThread();        Thread thread = new Thread(deamonThread);        thread.setDaemon(true);//设置为守护线程 且必须在start前调用        thread.start();        Scanner scanner = new Scanner(System.in);        scanner.next();//模拟阻塞        System.out.println("Main Thread Over!");    }}

运行结果：

file.txt文件内容：

使用jstack生成线程快照

运行程序后 打开任务管理器 找到当前程序的PID

打开CMD 输入

jstack -l pid

可以看到快照结果

我们可以分析一下

C:\Users\heqianqian>jstack -l 100922017-04-15 09:54:03Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.92-b14 mixed mode):"RMI RenewClean-[localhost:52353]" #18 daemon prio=5 os_prio=0 tid=0x00000000173ac000 nid=0x239c in Object.wait() [0x0000000017b0f000]   java.lang.Thread.State: TIMED_WAITING (on object monitor)        at java.lang.Object.wait(Native Method)        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)        - locked <0x00000000d01df4c0> (a java.lang.ref.ReferenceQueue$Lock)        at sun.rmi.transport.DGCClient$EndpointEntry$RenewCleanThread.run(DGCClient.java:553)        at java.lang.Thread.run(Thread.java:745)   Locked ownable synchronizers:        - None"RMI Scheduler(0)" #17 daemon prio=5 os_prio=0 tid=0x00000000173a9000 nid=0x255c waiting on condition [0x0000000017a0e000]   java.lang.Thread.State: TIMED_WAITING (parking)        at sun.misc.Unsafe.park(Native Method)        - parking to wait for  <0x00000000d02c2350> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)        at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2078)        at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1093)        at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)        at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)        at java.lang.Thread.run(Thread.java:745)   Locked ownable synchronizers:        - None"GC Daemon" #14 daemon prio=2 os_prio=-2 tid=0x000000001648b800 nid=0x2428 in Object.wait() [0x0000000016f0e000]   java.lang.Thread.State: TIMED_WAITING (on object monitor)        at java.lang.Object.wait(Native Method)        at sun.misc.GC$Daemon.run(GC.java:117)        - locked <0x00000000d02c2510> (a sun.misc.GC$LatencyLock)   Locked ownable synchronizers:        - None"RMI Reaper" #13 prio=5 os_prio=0 tid=0x0000000016607000 nid=0x2268 in Object.wait() [0x0000000016e0f000]   java.lang.Thread.State: WAITING (on object monitor)        at java.lang.Object.wait(Native Method)        - waiting on <0x00000000d01dfac0> (a java.lang.ref.ReferenceQueue$Lock)        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)        - locked <0x00000000d01dfac0> (a java.lang.ref.ReferenceQueue$Lock)        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)        at sun.rmi.transport.ObjectTable$Reaper.run(ObjectTable.java:351)        at java.lang.Thread.run(Thread.java:745)   Locked ownable synchronizers:        - None"RMI TCP Accept-0" #12 daemon prio=5 os_prio=0 tid=0x000000001655e000 nid=0x24a0 runnable [0x0000000016d0e000]   java.lang.Thread.State: RUNNABLE        at java.net.DualStackPlainSocketImpl.accept0(Native Method)        at java.net.DualStackPlainSocketImpl.socketAccept(DualStackPlainSocketImpl.java:131)        at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)        at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:199)        - locked <0x00000000d01df1b0> (a java.net.SocksSocketImpl)        at java.net.ServerSocket.implAccept(ServerSocket.java:545)        at java.net.ServerSocket.accept(ServerSocket.java:513)        at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.executeAcceptLoop(TCPTransport.java:400)        at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.run(TCPTransport.java:372)        at java.lang.Thread.run(Thread.java:745)   Locked ownable synchronizers:        - None"RMI TCP Accept-34855" #11 daemon prio=5 os_prio=0 tid=0x0000000016243800 nid=0x2338 runnable [0x0000000016c0f000]   java.lang.Thread.State: RUNNABLE        at java.net.DualStackPlainSocketImpl.accept0(Native Method)        at java.net.DualStackPlainSocketImpl.socketAccept(DualStackPlainSocketImpl.java:131)        at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)        at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:199)        - locked <0x00000000d0211318> (a java.net.SocksSocketImpl)        at java.net.ServerSocket.implAccept(ServerSocket.java:545)        at java.net.ServerSocket.accept(ServerSocket.java:513)        at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.executeAcceptLoop(TCPTransport.java:400)        at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.run(TCPTransport.java:372)        at java.lang.Thread.run(Thread.java:745)   Locked ownable synchronizers:        - None"Service Thread" #9 daemon prio=9 os_prio=0 tid=0x0000000016296000 nid=0x284c runnable [0x0000000000000000]   java.lang.Thread.State: RUNNABLE   Locked ownable synchronizers:        - None"C1 CompilerThread2" #8 daemon prio=9 os_prio=2 tid=0x000000001621b000 nid=0x27c0 waiting on condition [0x0000000000000000]   java.lang.Thread.State: RUNNABLE   Locked ownable synchronizers:        - None"C2 CompilerThread1" #7 daemon prio=9 os_prio=2 tid=0x0000000016215800 nid=0x286c waiting on condition [0x0000000000000000]   java.lang.Thread.State: RUNNABLE   Locked ownable synchronizers:        - None"C2 CompilerThread0" #6 daemon prio=9 os_prio=2 tid=0x0000000014c8d000 nid=0x26f0 waiting on condition [0x0000000000000000]   java.lang.Thread.State: RUNNABLE   Locked ownable synchronizers:        - None"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x0000000014c8b800 nid=0x27a0 waiting on condition [0x0000000000000000]   java.lang.Thread.State: RUNNABLE   Locked ownable synchronizers:        - None"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x0000000014c40000 nid=0x2780 runnable [0x0000000000000000]   java.lang.Thread.State: RUNNABLE   Locked ownable synchronizers:        - None"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x0000000002ebd000 nid=0x279c in Object.wait() [0x0000000015faf000]   java.lang.Thread.State: WAITING (on object monitor)        at java.lang.Object.wait(Native Method)        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)        - locked <0x00000000d02c32b0> (a java.lang.ref.ReferenceQueue$Lock)        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)   Locked ownable synchronizers:        - None"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x0000000002eb2000 nid=0x26e0 in Object.wait() [0x0000000015eaf000]   java.lang.Thread.State: WAITING (on object monitor)        at java.lang.Object.wait(Native Method)        at java.lang.Object.wait(Object.java:502)        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)        - locked <0x00000000d02ea1c8> (a java.lang.ref.Reference$Lock)        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)   Locked ownable synchronizers:        - None"main" #1 prio=5 os_prio=0 tid=0x0000000002dc2800 nid=0x2844 in Object.wait() [0x00000000029ff000]   java.lang.Thread.State: TIMED_WAITING (on object monitor)        at java.lang.Object.wait(Native Method)        at com.intellij.execution.rmi.RemoteServer.start(RemoteServer.java:86)        - locked <0x00000000d02c3350> (a java.lang.Object)        at org.jetbrains.idea.maven.server.RemoteMavenServer.main(RemoteMavenServer.java:22)   Locked ownable synchronizers:        - None"VM Thread" os_prio=2 tid=0x0000000014bf7000 nid=0x2794 runnable"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000002dd8800 nid=0x2878 runnable"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x0000000002dda000 nid=0x2804 runnable"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x0000000002ddb800 nid=0x1d9c runnable"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x0000000002ddd000 nid=0x25e8 runnable"VM Periodic Task Thread" os_prio=2 tid=0x00000000162dc000 nid=0x27c4 waiting on conditionJNI global references: 236C:\Users\heqianqian>

1. Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.92-b14 mixed mode)
   线程块运行在java虚拟机hotspot上 是64位的服务端
2. 接下来是一些java自带的守护进程
   比如GC线程

可以看到是deamon线程 优先级是2 当前状态为有时间的等待状态

Locked ownable synchronizers 是否处于同步块中

最后显示的是我们的主线程

可以看到主线程并不是一个守护线程 并且处于等待状态 原因是我们的键盘输入阻塞了主线程

jstack使用参数

参考文章：