Java 线程（一）：线程状态

 

监控工具

Java VisualVM 是可视化的查看JVM运行信息的工具，位于 $JAVA_HOME/bin/jvisualvm

官方文档 https://docs.oracle.com/javase/8/docs/technotes/guides/visualvm/

在菜单工具-插件，安装 Threads Inspector 插件可以查看线程运行状态。

 

线程状态

Thead的内部枚举类定义了JVM层面的6种线程状态。

• NEW

Thread state for a thread which has not yet started.

未启动的线程状态。

• RUNNABLE

Thread state for a runnable thread. A thread in the runnable state is executing in the Java virtual machine but it may be waiting for other resources from the operating system such as processor.

线程在JVM 中可运行，执行权交给 OS。

• BLOCKED

Thread state for a thread blocked waiting for a monitor lock. A thread in the blocked state is waiting for a monitor lock to enter a synchronized block/method or reenter a synchronized block/method after calling Object.wait.

线程等在获取 monitor lock，两类情况：一个线程等待 monitor lock 来进入同步代码块或同步方法；一个线程执行到同步代码块或者方法中的 Object.wait() 方法后处于 WAITING 状态，当其他线程调用 Object.notifyAll() 后再次尝试进入（reenter）同步代码块或方法时等待 monitor lock。

• WAITING

Thread state for a waiting thread. A thread is in the waiting state due to calling one of the following methods:

• • Object.wait with no timeout
  • Thread.join with no timeout
  • LockSupport.park

A thread in the waiting state is waiting for another thread to perform a particular action. For example, a thread that has called Object.wait() on an object is waiting for another thread to call Object.notify() or Object.notifyAll() on that object. A thread that has called Thread.join() is waiting for a specified thread to terminate.

JVM层面的等待或者阻塞。

• TIMED_WAITING
  • Thread state for a waiting thread with a specified waiting time. A thread is in the timed waiting state due to calling one of the following methods with a specified positive waiting time:
  • Thread.sleep
  • Object.wait with timeout
  • Thread.join with timeout
  • LockSupport.parkNanos
  • LockSupport.parkUntil
• TERMINATED

Thread state for a terminated thread. The thread has completed execution.

线程执行结束或者异常结束。

 

Blocked 和 Waiting 状态区别

可以简单理解线程为了对共享资源（同步代码块或者同步方法）的独占。

1. 获取不到 monitor lock 时进入 block set 处于 Blocked 状态。
2. 进入同步代码块后发现缺少某种资源，调用 Object.wait() 先释放共享资源（同步代码块或者同步方法）线程进入 wait set 处于 Waiting 状态，等待调用 Object.notifyAll() 呼唤自己出队，出队后重新尝试（reenter）获取共享资源（同步代码块或者同步方法）。

 

线程生命周期