查看java线程

╰+哭是因爲堅強的太久メ 2021-06-22 15:36 480阅读 0赞

我们知道java线程使用的是底层操作系统"轻量级线程"+"内核线程"的1:1模型，那么我们如何查看一个运行的java进程中线程数呢？

**1、linux上查看进程中线程数：**

主要有以下三种方法：

**1）top -Hp pid**

$ top -Hp 12121
    top - 13:37:23 up 223 days, 18:41,  2 users,  load average: 0.01, 0.02, 0.05
    Threads:  20 total,   0 running,  20 sleeping,   0 stopped,   0 zombie
    %Cpu(s):  0.1 us,  0.2 sy,  0.0 ni, 99.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
    KiB Mem : 16267876 total,   549064 free,  3220720 used, 12498092 buff/cache
    KiB Swap:  2097148 total,  2034684 free,    62464 used. 11924764 avail Mem 
    
      PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND                                  
    12121 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12122 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.10 java                                     
    12123 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12124 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12125 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12126 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12127 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12128 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12129 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12130 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12131 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12132 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12133 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12134 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12135 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12136 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12137 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12138 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12139 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.00 java                                     
    12140 root      20   0 6963564  26104  10532 S  0.0  0.2   0:00.03 java

**2）cat /proc/pid/status | grep Thread**

$ cat /proc/12121/status | grep Thread
    Threads: 20

**3）pstree -p pid：**

$ pstree -p 12121
    java(12121)─┬─{java}(12122)
                ├─{java}(12123)
                ├─{java}(12124)
                ├─{java}(12125)
                ├─{java}(12126)
                ├─{java}(12127)
                ├─{java}(12128)
                ├─{java}(12129)
                ├─{java}(12130)
                ├─{java}(12131)
                ├─{java}(12132)
                ├─{java}(12133)
                ├─{java}(12134)
                ├─{java}(12135)
                ├─{java}(12136)
                ├─{java}(12137)
                ├─{java}(12138)
                ├─{java}(12139)
                └─{java}(12140)

注：上述三种方法得到的结果都是一致的。

**2、通过java api打印java中的线程：**

import java.lang.management.ManagementFactory;
    import java.lang.management.ThreadInfo;
    import java.lang.management.ThreadMXBean;
    
    public class Test1 {
    	public static void main(String[] args) {
    		ThreadMXBean threadBean = ManagementFactory.getThreadMXBean();
    		ThreadInfo[] threadInfo = threadBean.dumpAllThreads(false, false);
    		for(ThreadInfo info : threadInfo){
                System.out.println(info.getThreadId() + "--" + info.getThreadName() +"--"+ info.getThreadState().name());
            }
    	}
    }

输出结果：

4--Signal Dispatcher--RUNNABLE

3--Finalizer--WAITING

2--Reference Handler--WAITING

1--main--RUNNABLE

说明：虽然我们写了一个main函数的单线程程序，但是JVM会启动一些额外的线程，主要包括：

*  Attach Listener 线程：负责接收外部JVM的命令，而对该命令进行执行的，并且把结果返回给发送者。如常用的命令：java -version、jmap、jstack等
 *  Signal Dispatcher 线程：Attach Listener线程接收到命令后，会交给Signal Dispatcher线程去进行分发到各个不同的模块处理命令，并且返回处理结果
 *  Finalizer 线程：在垃圾回收之前执行“对象完成”的Java系统线程
 *  Reference Handler  线程：处理引用对象本身（软引用、弱引用、虚引用）的垃圾回收问题

注意：Attach Listener、Signal Dispatcher线程在jvm启动的时候若没有初始化，那么则会在用户第一次执行jvm命令时启动这两个线程。

**3、实战示例：**

1）我们写一个main线程的（单线程）程序：

public class Test2 {
    	public static void main(String[] args) throws Exception {
    		while(true) {
    			Thread.sleep(2000);
        		System.out.println("main...");
    		}
    	}
    }

2）运行后，使用上面的方法查看该java进程的线程数：

可以看到该java进程包含了20个线程（使用另外的两种方法得到同样的结果）。这里有个问题，为什么会有这么多线程呢？

3）使用jstack查看线程详情：

$ jstack -l 12121
    2020-02-16 13:39:26
    Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.144-b01 mixed mode):
    
    "Attach Listener" #10 daemon prio=9 os_prio=0 tid=0x00002b9ad4001000 nid=0x3c78 waiting on condition [0x0000000000000000]
       java.lang.Thread.State: RUNNABLE
    
       Locked ownable synchronizers:
    	- None
    
    "Service Thread" #9 daemon prio=9 os_prio=0 tid=0x00002b9a800d2000 nid=0x2f6b runnable [0x0000000000000000]
       java.lang.Thread.State: RUNNABLE
    
       Locked ownable synchronizers:
    	- None
    
    "C1 CompilerThread3" #8 daemon prio=9 os_prio=0 tid=0x00002b9a800c7000 nid=0x2f6a waiting on condition [0x0000000000000000]
       java.lang.Thread.State: RUNNABLE
    
       Locked ownable synchronizers:
    	- None
    
    "C2 CompilerThread2" #7 daemon prio=9 os_prio=0 tid=0x00002b9a800c4800 nid=0x2f69 waiting on condition [0x0000000000000000]
       java.lang.Thread.State: RUNNABLE
    
       Locked ownable synchronizers:
    	- None
    
    "C2 CompilerThread1" #6 daemon prio=9 os_prio=0 tid=0x00002b9a800c3000 nid=0x2f68 waiting on condition [0x0000000000000000]
       java.lang.Thread.State: RUNNABLE
    
       Locked ownable synchronizers:
    	- None
    
    "C2 CompilerThread0" #5 daemon prio=9 os_prio=0 tid=0x00002b9a800c0000 nid=0x2f67 waiting on condition [0x0000000000000000]
       java.lang.Thread.State: RUNNABLE
    
       Locked ownable synchronizers:
    	- None
    
    "Signal Dispatcher" #4 daemon prio=9 os_prio=0 tid=0x00002b9a800be800 nid=0x2f66 runnable [0x0000000000000000]
       java.lang.Thread.State: RUNNABLE
    
       Locked ownable synchronizers:
    	- None
    
    "Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00002b9a8008b800 nid=0x2f65 in Object.wait() [0x00002b9abd8be000]
       java.lang.Thread.State: WAITING (on object monitor)
    	at java.lang.Object.wait(Native Method)
    	- waiting on <0x000000076d408ec8> (a java.lang.ref.ReferenceQueue$Lock)
    	at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
    	- locked <0x000000076d408ec8> (a java.lang.ref.ReferenceQueue$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=0 tid=0x00002b9a80086800 nid=0x2f64 in Object.wait() [0x00002b9abd7bd000]
       java.lang.Thread.State: WAITING (on object monitor)
    	at java.lang.Object.wait(Native Method)
    	- waiting on <0x000000076d406b68> (a java.lang.ref.Reference$Lock)
    	at java.lang.Object.wait(Object.java:502)
    	at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
    	- locked <0x000000076d406b68> (a java.lang.ref.Reference$Lock)
    	at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
    
       Locked ownable synchronizers:
    	- None
    
    "main" #1 prio=5 os_prio=0 tid=0x00002b9a80008800 nid=0x2f5a waiting on condition [0x00002b9a7b9c6000]
       java.lang.Thread.State: TIMED_WAITING (sleeping)
    	at java.lang.Thread.sleep(Native Method)
    	at Test2.main(Test2.java:5)
    
       Locked ownable synchronizers:
    	- None
    
    "VM Thread" os_prio=0 tid=0x00002b9a8007f000 nid=0x2f63 runnable 
    
    "GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00002b9a8001d800 nid=0x2f5b runnable 
    
    "GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00002b9a8001f800 nid=0x2f5c runnable 
    
    "GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00002b9a80021000 nid=0x2f5d runnable 
    
    "GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00002b9a80023000 nid=0x2f5e runnable 
    
    "GC task thread#4 (ParallelGC)" os_prio=0 tid=0x00002b9a80025000 nid=0x2f5f runnable 
    
    "GC task thread#5 (ParallelGC)" os_prio=0 tid=0x00002b9a80026800 nid=0x2f60 runnable 
    
    "GC task thread#6 (ParallelGC)" os_prio=0 tid=0x00002b9a80028800 nid=0x2f61 runnable 
    
    "GC task thread#7 (ParallelGC)" os_prio=0 tid=0x00002b9a8002a800 nid=0x2f62 runnable 
    
    "VM Periodic Task Thread" os_prio=0 tid=0x00002b9a800d5000 nid=0x2f6c waiting on condition 
    
    JNI global references: 6

在thread dump中每个线程都有一个nid（十六进制），该进成共有20个nid（线程）。主要包括：

*  Attach Listener、Signal Dispatcher、Finalizer、Reference Handler
 *  GC task thread：gc线程
 *  C1 CompilerThread：C1即时编译线程；
 *  C2 CompilerThread：C2即时编译线程；
 *  main：用户线程

4）修改java代码，添加一个线程：

public class Test {
    
    	public static void main(String[] args) throws Exception {
    		System.out.println("main...");
    
    		
    		new Thread() {
    			public void run() {
    				while(true) {
    					try {
    						Thread.sleep(1000);
    						System.out.println("test1...");
    					} catch(Exception e) {
    
    					}
    				}
    			}
    		}.start();
    		new Thread() {
    			public void run() {
    				while(true) {
    					try {
    						Thread.sleep(1000);
    						System.out.println("test2...");
    					} catch(Exception e) {
    
    					}
    				}
    			}
    		}.start();
    	}
    }

一共起了有3个线程（算main线程）。接下来我们运行该程序，并使用top查看该进程中的线程数。

$ top -Hp 11444
    top - 14:05:29 up 223 days, 19:09,  2 users,  load average: 0.00, 0.01, 0.05
    Threads:  22 total,   0 running,  22 sleeping,   0 stopped,   0 zombie
    %Cpu(s):  0.0 us,  0.1 sy,  0.0 ni, 99.9 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
    KiB Mem : 16267876 total,   537252 free,  3222404 used, 12508220 buff/cache
    KiB Swap:  2097148 total,  2034684 free,    62464 used. 11914756 avail Mem 
    
      PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND                                  
    11444 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11445 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.09 java                                     
    11447 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11448 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11449 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11450 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11451 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11452 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11453 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11454 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11466 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11473 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11477 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11487 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11488 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11489 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11490 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11491 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11492 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11493 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11494 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java                                     
    11495 root      20   0 7096692  26292  10624 S  0.0  0.2   0:00.00 java

我们看到一共有22个线程，比之前的单线程程序多了两个我们自定义的线程。使用jstack查看可以发现多了如下两个线程信息：

"Thread-1" #11 prio=5 os_prio=0 tid=0x00002adca410a000 nid=0x2ce7 waiting on condition [0x00002adce1715000]
       java.lang.Thread.State: TIMED_WAITING (sleeping)
    	at java.lang.Thread.sleep(Native Method)
    	at Test$2.run(Test.java:24)
    
       Locked ownable synchronizers:
    	- None
    
    "Thread-0" #10 prio=5 os_prio=0 tid=0x00002adca4108000 nid=0x2ce6 waiting on condition [0x00002adce1614000]
       java.lang.Thread.State: TIMED_WAITING (sleeping)
    	at java.lang.Thread.sleep(Native Method)
    	at Test$1.run(Test.java:12)
    
       Locked ownable synchronizers:
    	- None