因为之前自己基于HEXO搭建的个人博客很久没有维护，就把以前的文章搬到简书上来。“滥芋充数”的同时还能复习下以前的笔记。废话不多说，还是先看下jstack和jstat的简单使用吧。

jstack的用法

jstack是java虚拟机自带的一种堆栈跟踪工具。jstack用于打印出给定的java进程ID的Java堆栈信息。简单使用方法如下：

jstack pid

要看懂jstack输出的堆栈信息，首先要先知道堆栈信息中的名词的意思:

Waiting on monitor entry 和 in Object.wait()的详细描述：
Monitor是 Java中用于实现线程之间的互斥与协作的主要手段，它可以看成是对象或者 Class的锁，synchronized的底层就是基于monitor, 也就是monitor是sychronized的微观表现。每一个对象都有，也仅有一个 monitor。每个 Monitor在某个时刻，只能被一个线程拥有，该线程就是 “Active Thread”，而其它线程都是 “Waiting Thread”，分别在两个队列 “ Entry Set”和 “Wait Set”里面等候。在 “Entry Set”中等待的线程是 “Waiting for monitor entry”，而在 “Wait Set”中等待的线程是 “in Object.wait()”

blocked阻塞示例

public class JstackTest {
    
    private static final Object mutex = new Object();

    public static void main(String[] args) {
        final Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized(mutex) {
                    System.out.println(Thread.currentThread().getName());
                    try {
                        TimeUnit.SECONDS.sleep(60);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }
            
        });
        thread.setName("thread1");
        thread.start();
        synchronized (mutex) {
            System.out.println(Thread.currentThread().getName());
            try {
                TimeUnit.SECONDS.sleep(60);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

运行代码后,先输出主线程名main, 主线程sleep 60秒, thread1等待main线程释放锁,下面使用jstack分析线程的dump日志:

D:\java\gitlab\jvm-debug>jstack 16212
2020-06-10 16:41:35
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.172-b11 mixed mode):

"thread1" #12 prio=5 os_prio=0 tid=0x0000000020979800 nid=0x22dc waiting for monitor entry [0x000000002173f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at com.github.shizhengchao.stack.err.JstackTest$1.run(JstackTest.java:14)
        - waiting to lock <0x000000076b399b18> (a java.lang.Object)
        at java.lang.Thread.run(Thread.java:748)

.....
"main" #1 prio=5 os_prio=0 tid=0x0000000004d23800 nid=0x32ac waiting on condition [0x0000000004b3f000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at java.lang.Thread.sleep(Thread.java:340)
        at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)
        at com.github.shizhengchao.stack.err.JstackTest.main(JstackTest.java:30)
        - locked <0x000000076b399b18> (a java.lang.Object)

从dump文件里看到thread1 waiting for monitor entry等待获取锁对象0x000000076b399b18, 所以状态是BLOCKED,而锁对象0x000000076b399b18此时正在被main线程所占有,但没有释放,所以线程main状态为TIMED_WAITING，需要等待一个释放锁的条件触发（waiting on condition）.，我们设置的是sleep(60000)

deadlock死锁分析
死锁一般发生在重入锁里，两个线程互相占有对方需要的锁对象，都在等待对方释放锁。以下代码就是一个死锁,在多线程同时执行下可能发生死锁：

public class DeadLockThread extends Thread {
    
    private boolean flag;
    
    private static Object lock1 = new Object();
    private static Object lock2 = new Object();
    public DeadLockThread(boolean flag, String name) {
        this.flag = flag;
        this.setName(name);
    }

    @Override
    public void run() {
        if(flag) {
            synchronized(lock1) {
                System.out.println(Thread.currentThread().getName() + "获得取锁1");
                
                synchronized(lock2) {
                    System.out.println(Thread.currentThread().getName() + "获得取锁2");
                }
            }
        } else {
            synchronized(lock2) {
                System.out.println(Thread.currentThread().getName() + "获得取锁2");
                
                synchronized(lock1) {
                    System.out.println(Thread.currentThread().getName() + "获得取锁1");
                }
            }
        }
    }
}
// main里这样写：
DeadLockThread thread1 = new DeadLockThread(true, "thread1");
DeadLockThread thread2 = new DeadLockThread(false, "thread2");
thread1.start();
thread2.start();

运行代码后，执行jstack命令，发现了死锁：

D:\java\gitlab\jvm-debug>jstack 14116
2020-06-10 17:06:15
...
"thread2" #13 prio=5 os_prio=0 tid=0x0000000020b9f800 nid=0x29f4 waiting for monitor entry [0x0000000021abf000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at com.github.shizhengchao.stack.err.DeadLockThread.run(DeadLockThread.java:29)
        - waiting to lock <0x000000076b39bd60> (a java.lang.Object)
        - locked <0x000000076b39bd70> (a java.lang.Object)

"thread1" #12 prio=5 os_prio=0 tid=0x0000000020b9c800 nid=0xdd4 waiting for monitor entry [0x00000000219bf000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at com.github.shizhengchao.stack.err.DeadLockThread.run(DeadLockThread.java:21)
        - waiting to lock <0x000000076b39bd70> (a java.lang.Object)
        - locked <0x000000076b39bd60> (a java.lang.Object)
....
Found one Java-level deadlock:
=============================
"thread2":
  waiting to lock monitor 0x000000001eb00c38 (object 0x000000076b39bd60, a java.lang.Object),
  which is held by "thread1"
"thread1":
  waiting to lock monitor 0x000000001eb03578 (object 0x000000076b39bd70, a java.lang.Object),
  which is held by "thread2"

Java stack information for the threads listed above:
===================================================
"thread2":
        at com.github.shizhengchao.stack.err.DeadLockThread.run(DeadLockThread.java:29)
        - waiting to lock <0x000000076b39bd60> (a java.lang.Object)
        - locked <0x000000076b39bd70> (a java.lang.Object)
"thread1":
        at com.github.shizhengchao.stack.err.DeadLockThread.run(DeadLockThread.java:21)
        - waiting to lock <0x000000076b39bd70> (a java.lang.Object)
        - locked <0x000000076b39bd60> (a java.lang.Object)

Found 1 deadlock.

很显然，jstack已经为我们找到了一处死锁：Found 1 deadlock.线程thread2等待的monitor 0x000000001eb00c38 被thead1占用，线程hread1等待的monitor 0x000000001eb03578被thread2占用。

jstat的用法

jstat命令可以查看堆内存各部分的使用量，以及加载类的数量。
命令的格式如下:

jstat [-命令选项] [pid] [间隔时间/毫秒] [查询次数]

类加载统计

C:\Users\Administrator> jstat -class 35012
Loaded  Bytes  Unloaded  Bytes  Time
  430   881.6   0     0.0   0.07

垃圾回收统计

C:\Users\Administrator> jstat -gc 35012
 S0C   S1C   S0U  S1U  EC      EU    OC     OU   MC   MU    CCSC  CCSU   YGC   YGCT  FGC FGCT   GCT
5120.0 5120.0  0.0  0.0  33280.0  3328.4  87552.0  0.0  4480.0 781.2  384.0   75.9   0    0.000  0   0.000  0.000

总结垃圾回收统计

C:\Users\Administrator> jstat -gcutil 35012
  S0   S1   E     O   M    CCS   YGC    YGCT   FGC   FGCT    GCT
 0.00  0.00  10.00  0.00  17.44  19.76   0    0.000   0    0.000   0.000