Jvmtoolstudy 学习本文中JVM工具，协助定位问题

jstat -help
jstat -options
jstat -snap <pid>
jstat -<option> [-t] [-h<lines>] <pid> [<mill second> [<count>]]

jmap -help
jmap -dump
jmap -histo

 echo 'ibase=10;obase=16;3845' | bc

1.运行BusyThreadTest
2.jps
3.jstack -l <pid>

"BusyThread-99" prio=5 tid=0x00007fd2d2839000 nid=0x11603 waiting on condition [0x000000011e4d7000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
	at java.lang.Thread.sleep(Native Method)
	at com.jimmysc.thread.busy.BusyThread1.readFileFormRemote(BusyThread1.java:18)
	at com.jimmysc.thread.busy.BusyThread1.run(BusyThread1.java:13)
	at java.lang.Thread.run(Thread.java:744)

   Locked ownable synchronizers:
	- None

"BusyThread-98" prio=5 tid=0x00007fd2d2838000 nid=0x11403 waiting on condition [0x000000011e3d4000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
	at java.lang.Thread.sleep(Native Method)
	at com.jimmysc.thread.busy.BusyThread1.readFileFormRemote(BusyThread1.java:18)
	at com.jimmysc.thread.busy.BusyThread1.run(BusyThread1.java:13)
	at java.lang.Thread.run(Thread.java:744)

所有的线程都在执行com.jimmysc.thread.busy.BusyThread1.readFileFormRemote(BusyThread1.java:18)
说明该方法处理速度过慢，这种情况下会整个应用的线程逐步被耗光，导致应用的处理能力急剧下降

1.运行ThreadDeadLockTest
2.JPS
3.jstack -l <pid>

"Thread-0" prio=5 tid=0x00007f94f3832000 nid=0x5003 waiting for monitor entry [0x000000011972f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.jimmysc.thread.deadlock.Thread1.run(Thread1.java:19)
- waiting to lock <0x00000007d57a7008> (a java.lang.Class for com.jimmysc.thread.deadlock.ThreadLock2)
- locked <0x00000007d5655148> (a java.lang.Class for com.jimmysc.thread.deadlock.ThreadLock1)
at java.lang.Thread.run(Thread.java:744)

Locked ownable synchronizers:
- None
线程有3部分信息
1.线程的状态：waiting for monitor entry
2.线程的调用栈：
3.线程当前锁住的资源：locked <0x00000007d5655148> (a java.lang.Class for com.jimmysc.thread.deadlock.ThreadLock1)

1.runnable
	表示线程具备运行条件等待操作系统调度，或正在运行中
2.waiting on condition
	表示线程等待某个条件的发生：比如sleep、网络空闲等
3.Waiting for monitor entry 和 in Object.wait() 
	Monitor是java中实现线程之间互斥和协作的主要手段，它可以看成是对象或class的锁，每个对象都有，但只有一个Monitor，下图描述Monitor，线程及线程状态的转换图

任何时刻，每个 Monitor只能被 “Active Thread”拥有。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”中等待的线程状态是 “Waiting for monitor entry”，在 “Wait Set”中等待的线程状态是 “in Object.wait()”
当执行到synchronized块的代码时（临界区），线程进入Entry Set队列中，若此时Monitor不被其他线程占用，并且Entry Set中没有其他线程，则当前线程会成为Monitor的Owner，执行synchronized中的代码，否则会一直在Entry Set等待；如果线程调用wait方法，则线程进入Wait Set等待，除非线程调用了对象的notify() 或 notifyAll()，Wait Set中的线程才有机会去竞争Monitor，但最终只有一个线程获得Monitor

	"Thread-1" prio=5 tid=0x00007f94f3832800 nid=0x5203 waiting for monitor entry [0x0000000119832000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.jimmysc.thread.deadlock.Thread2.run(Thread2.java:19)
	- waiting to lock <0x00000007d5655148> (a java.lang.Class for com.jimmysc.thread.deadlock.ThreadLock1)
	- locked <0x00000007d57a7008> (a java.lang.Class for com.jimmysc.thread.deadlock.ThreadLock2)
	at java.lang.Thread.run(Thread.java:744)

   Locked ownable synchronizers:
	- None

"Thread-0" prio=5 tid=0x00007f94f3832000 nid=0x5003 waiting for monitor entry [0x000000011972f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.jimmysc.thread.deadlock.Thread1.run(Thread1.java:19)
	- waiting to lock <0x00000007d57a7008> (a java.lang.Class for com.jimmysc.thread.deadlock.ThreadLock2)
	- locked <0x00000007d5655148> (a java.lang.Class for com.jimmysc.thread.deadlock.ThreadLock1)
	at java.lang.Thread.run(Thread.java:744)

   Locked ownable synchronizers:
	- None	

1).Thread-1和Thread-0的线程状态都是waiting for monitor entry，表示都是在Entry Set队列中，都在等待拿到Monitor，多执行几次jstack -l <pid> 线程栈的信息都一样，说明无法拿到monitor；
2).线程栈的提示信息显示BLOCKED，同时Thread-0在waiting to lock <0x00000007d57a7008> ；Thread-0又locked <0x00000007d5655148> ，Thread-1在waiting to lock <0x00000007d5655148>，locked <0x00000007d57a7008>，都锁住对方需要的锁，同时又等待对方释放锁，造成了死锁