線程的狀態(tài)

zhou_you 發(fā)布于2019-08-16 16:07 / 2080人閱讀

摘要：運(yùn)行中，線程狀態(tài)中并沒有這一狀態(tài)，但是實(shí)際執(zhí)行中是有的可運(yùn)行狀態(tài)的線程獲得了時間片，執(zhí)行程序代碼。其他阻塞運(yùn)行的線程發(fā)出了請求時，會把該線程置為阻塞狀態(tài)。當(dāng)處理完畢時，線程重新轉(zhuǎn)入可運(yùn)行狀態(tài)。

Thread.State

首先看JDK中的代碼: java.lang.Thread.State

/** * A thread state. A thread can be in one of the following states: * 一個線程的狀態(tài)，一個線程可以處于以下狀態(tài)中的某一個狀態(tài) *

{@link #NEW}
* A thread that has not yet started is in this state. *
{@link #RUNNABLE}
* A thread executing in the Java virtual machine is in this state. *
{@link #BLOCKED}
* A thread that is blocked waiting for a monitor lock * is in this state. *
{@link #WAITING}
* A thread that is waiting indefinitely for another thread to * perform a particular action is in this state. *
{@link #TIMED_WAITING}
* A thread that is waiting for another thread to perform an action * for up to a specified waiting time is in this state. *
{@link #TERMINATED}
* A thread that has exited is in this state. *

* *

* A thread can be in only one state at a given point in time. * These states are virtual machine states which do not reflect * any operating system thread states. * * @since 1.5 * @see #getState */ public enum State { /** * Thread state for a thread which has not yet started. */ NEW, ? /** * 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. */ RUNNABLE, ? /** * 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 * {@link Object#wait() Object.wait}. */ BLOCKED, ? /** * Thread state for a waiting thread. * A thread is in the waiting state due to calling one of the * following methods: *

{@link Object#wait() Object.wait} with no timeout
{@link #join() Thread.join} with no timeout
{@link LockSupport#park() 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. */ 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: *

{@link #sleep Thread.sleep}
{@link Object#wait(long) Object.wait} with timeout
{@link #join(long) Thread.join} with timeout
{@link LockSupport#parkNanos LockSupport.parkNanos}
{@link LockSupport#parkUntil LockSupport.parkUntil}

*/ TIMED_WAITING, ? /** * Thread state for a terminated thread. * The thread has completed execution. */ TERMINATED; }

1、新建(New)

新創(chuàng)建了一個線程對象，還未調(diào)用start()方法。

2、就緒（Runnable）

線程對象創(chuàng)建后，其他線程(比如main線程）調(diào)用了該對象的start()方法。該狀態(tài)的線程位于可運(yùn)行線程池中，等待被線程調(diào)度選中獲取cpu 的使用權(quán) 。

3、運(yùn)行中（Running，線程狀態(tài)中并沒有這一狀態(tài)，但是實(shí)際執(zhí)行中是有的）

可運(yùn)行狀態(tài)(runnable)的線程獲得了cpu 時間片（timeslice），執(zhí)行程序代碼。

4、限期等待（Timed Waiting）

也可以稱作 TIMED_WAITING（有等待時間的等待狀態(tài)）。

線程主動調(diào)用以下方法：

Thread.sleep方法；

Object的wait方法，帶有時間；

Thread.join方法，帶有時間；

LockSupport的parkNanos方法，帶有時間。

5、無限期等待（Waiting）

運(yùn)行中（Running）的線程執(zhí)行了以下方法：

Object的wait方法，并且沒有使用timeout參數(shù);

Thread的join方法，沒有使用timeout參數(shù)；

LockSupport的park方法；

Conditon的await方法。

6、阻塞（Blocked）

阻塞狀態(tài)是指線程因為某種原因放棄了cpu 使用權(quán)，暫時停止運(yùn)行。直到線程進(jìn)入可運(yùn)行(runnable)狀態(tài)，才有機(jī)會再次獲得cpu timeslice 轉(zhuǎn)到運(yùn)行(running)狀態(tài)。阻塞的情況分兩種：

同步阻塞：運(yùn)行(running)的線程進(jìn)入了一個synchronized方法，若該同步鎖被別的線程占用，則JVM會把該線程放入鎖池(lock pool)中。

其他阻塞：運(yùn)行(running)的線程發(fā)出了I/O請求時，JVM會把該線程置為阻塞狀態(tài)。當(dāng)I/O處理完畢時，線程重新轉(zhuǎn)入可運(yùn)行(runnable)狀態(tài)。

7、結(jié)束（Terminated）

線程run()、main() 方法執(zhí)行結(jié)束，或者因異常退出了run()方法，則該線程結(jié)束生命周期。

線程狀態(tài)探秘

jstack查看線程狀態(tài)
jstack -l 即可察看線程狀態(tài)，如何使用呢？

隨便寫一個死循環(huán)看一下

public class TestThreadState {
    public static void main(String[] args) {
        for (; ; ) {
?
        }
    }
}

ps -ef|grep TestThreadState，找到對應(yīng)的pid，jstack -l 即可，如果未輸出線程信息，可以嘗試使用-F參數(shù)來強(qiáng)制輸出。

"main" #1 prio=5 os_prio=31 tid=0x00007f8194801800 nid=0x1603 runnable [0x000070000a9b4000]
   java.lang.Thread.State: RUNNABLE
        at org.java.bin.TestThreadState.main(TestThreadState.java:12)