線程、多線程和線程池，看完這些你就能全部搞懂了

摘要：一文帶你搞懂線程、多線程和線程池。

一.線程

在操作系統中，線程是比進程更小的能夠獨立運行的基本單位。同時，它也是CPU調度的基本單位。線程本身基本上不擁有系統資源，只是擁有一些在運行時需要用到的系統資源，例如程序計數器，寄存器和棧等。一個進程中的所有線程可以共享進程中的所有資源。

二.多線程

多線程可以理解為在同一個程序中能夠同時運行多個不同的線程來執行不同的任務，這些線程可以同時利用CPU的多個核心運行。多線程編程能夠最大限度的利用CPU的資源。如果某一個線程的處理不需要占用CPU資源時（例如IO線程），可以使當前線程讓出CPU資源來讓其他線程能夠獲取到CPU資源，進而能夠執行其他線程對應的任務，達到最大化利用CPU資源的目的。

三.線程的實現方式

在Java中，實現線程的方式大體上分為三種，通過繼承Thread類、實現Runnable接口，實現Callable接口。簡單的示例代碼分別如下所示。

1）繼承Thread類代碼

package io.binghe.concurrent.executor.test;
/**
 * @author binghe
 * @version 1.0.0
 * @description 繼承Thread實現線程
 */
public class ThreadTest extends Thread {
    @Override
    public void run() {
        //TODO 在此寫在線程中執行的業務邏輯
    }
}

2）實現Runnable接口的代碼

package io.binghe.concurrent.executor.test;
/**
 * @author binghe
 * @version 1.0.0
 * @description 實現Runnable實現線程
 */
public class RunnableTest implements Runnable {
    @Override
    public void run() {
        //TODO 在此寫在線程中執行的業務邏輯
    }
}

3）實現Callable接口的代碼

package io.binghe.concurrent.executor.test;
​
import java.util.concurrent.Callable;
​
/**
 * @author binghe
 * @version 1.0.0
 * @description 實現Callable實現線程
 */
public class CallableTest implements Callable<String> {
    @Override
    public String call() throws Exception {
        //TODO 在此寫在線程中執行的業務邏輯
        return null;
    }
}

四.線程的生命周期

1）生命周期

一個線程從創建，到最終的消亡，需要經歷多種不同的狀態，而這些不同的線程狀態，由始至終也構成了線程生命周期的不同階段。線程的生命周期可以總結為下圖。

其中，幾個重要的狀態如下所示。

• NEW：初始狀態，線程被構建，但是還沒有調用start()方法。
• RUNNABLE：可運行狀態，可運行狀態可以包括：運行中狀態和就緒狀態。
• BLOCKED：阻塞狀態，處於這個狀態的線程需要等待其他線程釋放鎖或者等待進入synchronized。
• WAITING：表示等待狀態，處於該狀態的線程需要等待其他線程對其進行通知或中斷等操作，進而進入下一個狀態。
• TIME_WAITING：超時等待狀態。可以在一定的時間自行返回。
• TERMINATED：終止狀態，當前線程執行完畢。

2）代碼示例

為了更好的理解線程的生命周期，以及生命周期中的各個狀態，接下來使用代碼示例來輸出線程的每個狀態信息。

• WaitingTime

創建WaitingTime類，在while(true)循環中調用TimeUnit.SECONDS.sleep(long)方法來驗證線程的TIMED_WARTING狀態，代碼如下所示。

package io.binghe.concurrent.executor.state;
import java.util.concurrent.TimeUnit;
​
/**
 * @author binghe
 * @version 1.0.0
 * @description 線程不斷休眠
 */
public class WaitingTime implements Runnable{
    @Override
    public void run() {
        while (true){
            waitSecond(200);
        }
    }
    //線程等待多少秒
    public static final void waitSecond(long seconds){
        try {
            TimeUnit.SECONDS.sleep(seconds);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

• WaitingState

創建WaitingState類，此線程會在一個while(true)循環中，獲取當前類Class對象的synchronized鎖，也就是說，這個類無論創建多少個實例，synchronized鎖都是同一個，並且線程會處於等待狀態。接下來，在synchronized中使用當前類的Class對象的wait()方法，來驗證線程的WAITING狀態，代碼如下所示。

package io.binghe.concurrent.executor.state;
/**
 * @author binghe
 * @version 1.0.0
 * @description 線程在Warting上等待
 */
public class WaitingState implements Runnable {
    @Override
    public void run() {
        while (true){
            synchronized (WaitingState.class){
                try {
                    WaitingState.class.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

• BlockedThread

BlockedThread主要是在synchronized代碼塊中的while(true)循環中調用TimeUnit.SECONDS.sleep(long)方法來驗證線程的BLOCKED狀態。當啟動兩個BlockedThread線程時，首先啟動的線程會處於TIMED_WAITING狀態，后啟動的線程會處於BLOCKED狀態。代碼如下所示。

package io.binghe.concurrent.executor.state;
/**
 * @author binghe
 * @version 1.0.0
 * @description 加鎖后不再釋放鎖
 */
public class BlockedThread implements Runnable {
    @Override
    public void run() {
        synchronized (BlockedThread.class){
            while (true){
                WaitingTime.waitSecond(100);
            }
        }
    }
}

• ThreadState

啟動各個線程，驗證各個線程輸出的狀態，代碼如下所示。

package io.binghe.concurrent.executor.state;
​
/**
 * @author binghe
 * @version 1.0.0
 * @description 線程的各種狀態，測試線程的生命周期
 */
public class ThreadState {
​
    public static void main(String[] args){
        new Thread(new WaitingTime(), "WaitingTimeThread").start();
        new Thread(new WaitingState(), "WaitingStateThread").start();
​
        //BlockedThread-01線程會搶到鎖，BlockedThread-02線程會阻塞
        new Thread(new BlockedThread(), "BlockedThread-01").start();
        new Thread(new BlockedThread(), "BlockedThread-02").start();
    }
}

運行ThreadState類，如下所示。

可以看到，未輸出任何結果信息。可以在命令行輸入“jps”命令來查看運行的Java進程。

c:\>jps
21584 Jps
17828 KotlinCompileDaemon
12284 Launcher
24572
28492 ThreadState

可以看到ThreadSate進程的進程號為28492，接下來，輸入“jstack 28492”來查看ThreadSate進程棧的信息，如下所示。

c:\>jstack 28492
2020-02-15 00:27:08
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.202-b08 mixed mode):
​
"DestroyJavaVM" #16 prio=5 os_prio=0 tid=0x000000001ca05000 nid=0x1a4 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
​
"BlockedThread-02" #15 prio=5 os_prio=0 tid=0x000000001ca04800 nid=0x6eb0 waiting for monitor entry [0x000000001da4f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at io.binghe.concurrent.executor.state.BlockedThread.run(BlockedThread.java:28)
        - waiting to lock <0x0000000780a7e4e8> (a java.lang.Class for io.binghe.concurrent.executor.state.BlockedThread)
        at java.lang.Thread.run(Thread.java:748)
​
"BlockedThread-01" #14 prio=5 os_prio=0 tid=0x000000001ca01800 nid=0x6e28 waiting on condition [0x000000001d94f000]
   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 io.binghe.concurrent.executor.state.WaitingTime.waitSecond(WaitingTime.java:36)
        at io.binghe.concurrent.executor.state.BlockedThread.run(BlockedThread.java:28)
        - locked <0x0000000780a7e4e8> (a java.lang.Class for io.binghe.concurrent.executor.state.BlockedThread)
        at java.lang.Thread.run(Thread.java:748)
​
"WaitingStateThread" #13 prio=5 os_prio=0 tid=0x000000001ca06000 nid=0x6fe4 in Object.wait() [0x000000001d84f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x0000000780a7b488> (a java.lang.Class for io.binghe.concurrent.executor.state.WaitingState)
        at java.lang.Object.wait(Object.java:502)
        at io.binghe.concurrent.executor.state.WaitingState.run(WaitingState.java:29)
        - locked <0x0000000780a7b488> (a java.lang.Class for io.binghe.concurrent.executor.state.WaitingState)
        at java.lang.Thread.run(Thread.java:748)
​
"WaitingTimeThread" #12 prio=5 os_prio=0 tid=0x000000001c9f8800 nid=0x3858 waiting on condition [0x000000001d74f000]
   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 io.binghe.concurrent.executor.state.WaitingTime.waitSecond(WaitingTime.java:36)
        at io.binghe.concurrent.executor.state.WaitingTime.run(WaitingTime.java:29)
        at java.lang.Thread.run(Thread.java:748)
​
"Service Thread" #11 daemon prio=9 os_prio=0 tid=0x000000001c935000 nid=0x6864 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
​
"C1 CompilerThread3" #10 daemon prio=9 os_prio=2 tid=0x000000001c88c800 nid=0x6a28 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
​
"C2 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x000000001c880000 nid=0x6498 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
​
"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x000000001c87c000 nid=0x693c waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
​
"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x000000001c87b800 nid=0x5d00 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
​
"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x000000001c862000 nid=0x6034 runnable [0x000000001d04e000]
   java.lang.Thread.State: RUNNABLE
        at java.net.SocketInputStream.socketRead0(Native Method)
        at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
        at java.net.SocketInputStream.read(SocketInputStream.java:171)
        at java.net.SocketInputStream.read(SocketInputStream.java:141)
        at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
        at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
        at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
        - locked <0x0000000780b2fd88> (a java.io.InputStreamReader)
        at java.io.InputStreamReader.read(InputStreamReader.java:184)
        at java.io.BufferedReader.fill(BufferedReader.java:161)
        at java.io.BufferedReader.readLine(BufferedReader.java:324)
        - locked <0x0000000780b2fd88> (a java.io.InputStreamReader)
        at java.io.BufferedReader.readLine(BufferedReader.java:389)
        at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:64)
​
"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x000000001c788800 nid=0x6794 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
​
"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x000000001c7e3800 nid=0x3354 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
​
"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x000000001c771000 nid=0x6968 in Object.wait() [0x000000001cd4f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x0000000780908ed0> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
        - locked <0x0000000780908ed0> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:216)
​
"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x000000001c770800 nid=0x6590 in Object.wait() [0x000000001cc4f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x0000000780906bf8> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:502)
        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
        - locked <0x0000000780906bf8> (a java.lang.ref.Reference$Lock)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
​
"VM Thread" os_prio=2 tid=0x000000001a979800 nid=0x5c2c runnable
​
"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00000000033b9000 nid=0x4dc0 runnable
​
"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00000000033ba800 nid=0x6690 runnable
​
"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00000000033bc000 nid=0x30b0 runnable
​
"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00000000033be800 nid=0x6f68 runnable
​
"GC task thread#4 (ParallelGC)" os_prio=0 tid=0x00000000033c1000 nid=0x6478 runnable
​
"GC task thread#5 (ParallelGC)" os_prio=0 tid=0x00000000033c2000 nid=0x4fe4 runnable
​
"GC task thread#6 (ParallelGC)" os_prio=0 tid=0x00000000033c5000 nid=0x584 runnable
​
"GC task thread#7 (ParallelGC)" os_prio=0 tid=0x00000000033c6800 nid=0x6988 runnable
​
"VM Periodic Task Thread" os_prio=2 tid=0x000000001c959800 nid=0x645c waiting on condition
​
JNI global references: 12

由以上輸出的信息可以看出：名稱為WaitingTimeThread的線程處於TIMED_WAITING狀態；名稱為WaitingStateThread的線程處於WAITING狀態；名稱為BlockedThread-01的線程處於TIMED_WAITING狀態；名稱為BlockedThread-02的線程處於BLOCKED狀態。

注意：使用jps結合jstack命令可以分析線上生產環境的Java進程的異常信息。

也可以直接點擊IDEA下圖所示的圖表直接打印出線程的堆棧信息。

輸出的結果信息與使用“jstack 進程號”命令輸出的信息基本一致。

 本文分享自華為雲社區《一文搞懂線程與線程池》，原文作者：冰 河。

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