Detailed explanation of Java multithreading

Introduction to multi-threading background knowledge

Using multi-threading can simplify the model and write powerful code, but it is not easy to write multi-threading well and requires a long-term practice process.

Introduction to the basic concepts of multi-threading

Processes and threads

① Process: the execution process of a program (task). Dynamic

Holding resources (shared memory, shared files) and threads. Carrier

Example: Eclipse, QQ

② Thread:

Eclipse: source code text editing, source code compilation, syntax verification.

QQ: text chat, send and receive files.

If the process is compared to a class, then every student in this class is a thread. Students are the smallest unit in the class and constitute the smallest unit in the class. A class can have multiple students, and these students all use the same tables, chairs, blackboards, and chalk in the class.

Thread is the smallest execution unit in the system. There are multiple threads in the same process, and threads share the resources of the process.

Interaction of threads

Mutual exclusion and synchronization.

First experience with Java thread

Java.lang

class Thread

interface Runnable

public void run ()

Common methods of threads

##static void yield()The currently running thread releases processor resourcesGets thread referencestatic Thread currentThread()Returns the currently running thread reference

When the two threads are not doing any processing, they will run alternately.

When using the boolean type to control the loop of a thread, add the volatile keyword before the variable. Volatile ensures that the thread can correctly read the values ​​written by other threads.

Note:

The function of the sleep() method: makes the thread sleep for the specified time.

The function of the join() method: //Make other threads wait for the current thread to complete execution.

Multi-threading case

Example 1:

1 package com.czgo;  
2   
3    
4   
5 /**  
6   
7  * 线程先生  
8   
9  * @author 疯子 
10  
11  * 
12  
13  */ 
14  
15 public class Actor extends Thread { 
16  
17     @Override 
18  
19     public void run() { 
20  
21         //getName()：获取当前线程的名称 
22  
23         System.out.println(getName()+"是一个演员！"); 
24  
25         //用来记录线程跑的次数 
26  
27         int count = 0; 
28  
29         boolean keepRunning = true; 
30  
31         while(keepRunning){ 
32  
33             System.out.println(getName()+"登台演出"+(++count)); 
34  
35             if(count==100){ 
36  
37                 keepRunning = false; 
38  
39             } 
40  
41             if(count%10==0){ 
42  
43                 try { 
44  
45                     Thread.sleep(1000); 
46  
47                 } catch (InterruptedException e) { 
48  
49                     e.printStackTrace(); 
50  
51                 } 
52  
53             } 
54  
55         } 
56  
57         System.out.println(getName()+"的演出结束了！"); 
58  
59     } 
60  
61     
62  
63     public static void main(String[] args) { 
64  
65         Thread actor = new Actor(); 
66  
67         //setName：设置线程的名称 
68  
69         actor.setName("Mr.Thread"); 
70  
71         //启动线程 
72  
73         actor.start(); 
74  
75         
76  
77         Thread actressThread = new Thread(new Actress(),"Ms.Runnable"); 
78  
79         actressThread.start(); 
80  
81     } 
82  
83 } 
84  
85   
86  
87 class Actress implements Runnable{ 
88  
89     @Override 
90  
91     public void run() { 
92  
93         //getName()：获取当前线程的名称 
94  
95         //currentThread()获取当前线程的引用 
96  
97         System.out.println(Thread.currentThread().getName()+"是一个演员！"); 
98  
99         //用来记录线程跑的次数
100 
101         int count = 0;
102 
103         boolean keepRunning = true;
104 
105         while(keepRunning){
106 
107             System.out.println(Thread.currentThread().getName()+"登台演出"+(++count));
108 
109             if(count==100){
110 
111                 keepRunning = false;
112 
113             }
114 
115             if(count%10==0){
116 
117                 try {
118 
119                     Thread.sleep(1000);
120 
121                 } catch (InterruptedException e) {
122 
123                     e.printStackTrace();
124 
125                 }
126 
127             }
128 
129         }
130 
131         System.out.println(Thread.currentThread().getName()+"的演出结束了！");   
132 
133     }134 135 }

Example 2:

Army:

 1 package com.czgo; 
 2  
 3   
 4  
 5 /** 
 6  
 7  * 军队线程 
 8  
 9  * 模拟作战双方的行为
 10 
 11  * @author 疯子
 12 
 13  *
 14 
 15  */
 16 
 17 public class ArmyRunnable implements Runnable {
 18 
 19    
 20 
 21     //volatile保证了线程可以正确的读取其他线程写入的值
 22 
 23     //可见性 ref JMM,happens-before
 24 
 25     volatile boolean keepRunning = true;
 26 
 27    
 28 
 29     @Override
 30 
 31     public void run() {
 32 
 33        
 34 
 35         while(keepRunning){
 36 
 37             //发动5连击
 38 
 39             for(int i=0;i<5;i++){
 40 
 41                 System.out.println(Thread.currentThread().getName()+"进攻对方["+i+"]");
 42 
 43                 //让出了处理器时间，下次谁进攻还不一定呢！
 44 
 45                 Thread.yield();
 46 
 47             }
 48 
 49         }
 50 
 51        
 52 
 53         System.out.println(Thread.currentThread().getName()+"结束了战斗！");
 54 
 55        
 56 
 57     }
 58 
 59 }

Key Character:

1 package com.czgo; 
2  
3   
4  
5 /** 
6  
7  * 关键人物 
8  
9  * @author 疯子
10 
11  *
12 
13  */
14 
15 public class KeyPersonThread extends Thread {
16 
17     @Override
18 
19     public void run() {
20 
21         System.out.println(Thread.currentThread().getName()+"开始了战斗！"); 
22 
23         for(int i=0;i<10;i++){
24 
25             System.out.println(Thread.currentThread().getName()+"左突右杀，攻击随军...");
26 
27         }
28 
29         System.out.println(Thread.currentThread().getName()+"结束了战斗！");
30 
31  
32 
33     }
34 
35 }

Stage:

  1 package com.czgo;  
  2   
  3    
  4   
  5 /**  
  6   
  7  * 隋唐演义大戏舞台  
  8   
  9  * @author win7 
  10  
  11  * 
  12  
  13  */ 
  14  
  15 public class Stage extends Thread { 
  16  
  17     
  18  
  19     @Override 
  20  
  21     public void run() { 
  22  
  23         System.out.println("欢迎观看隋唐演义"); 
  24  
  25         
  26  
  27         try { 
  28  
  29             Thread.sleep(5000); 
  30  
  31         } catch (InterruptedException e2) { 
  32  
  33             e2.printStackTrace(); 
  34  
  35         } 
  36  
  37         
  38  
  39         System.out.println("大幕徐徐拉开"); 
  40  
  41         
  42  
  43         try { 
  44  
  45             Thread.sleep(5000); 
  46  
  47         } catch (InterruptedException e2) { 
  48  
  49             e2.printStackTrace(); 
  50  
  51         } 
  52 
  53         
  54  
  55         System.out.println("话说隋朝末年，隋军与农民起义军杀得昏天暗地..."); 
  56  
  57         
  58  
  59         //隋朝军队 
  60  
  61         ArmyRunnable armyTaskOfSuiDynasty = new ArmyRunnable(); 
  62 
  63         //农民起义军 
  64  
  65         ArmyRunnable armyTaskOfRevolt = new ArmyRunnable(); 
  66  
  67         
  68  
  69         //使用Runnable接口创建线程 
  70  
  71         Thread armyOfSuiDynasty = new Thread(armyTaskOfSuiDynasty,"隋军"); 
  72  
  73         Thread armyOfSuiRevolt = new Thread(armyTaskOfRevolt,"农民起义军"); 
  74  
  75         
  76  
  77         //启动线程，让军队开始作战 
  78  
  79         armyOfSuiDynasty.start(); 
  80  
  81         armyOfSuiRevolt.start(); 
  82  
  83         
  84  
  85         //舞台线程休眠，大家专心观看军队的厮杀 
  86  
  87         try { 
  88  
  89             //Thread会指向当前类的线程 
  90 
  91             Thread.sleep(50); 
  92  
  93         } catch (InterruptedException e) { 
  94  
  95             e.printStackTrace(); 
  96  
  97         } 
  98
  99         System.out.println("正当双方激战正酣，半路杀出了个程咬金");
  100 
  101        
  102 
  103         Thread mrCheng = new KeyPersonThread();
  104 
  105         mrCheng.setName("程咬金");
  106 
  107        
  108 
  109         System.out.println("程咬金的理想就是结束战争，使百姓安居乐业！");
  110 
  111        
  112 
  113         //停止军队作战
  114 
  115         //停止线程的方法
  116 
  117         armyTaskOfSuiDynasty.keepRunning=false;
  118 
  119         armyTaskOfRevolt.keepRunning=false;
  120 
  121       
  122 
  123         try {
  124 
  125             Thread.sleep(2000);
  126 
  127         } catch (InterruptedException e1) {
  128 
  129             e1.printStackTrace();
  130 
  131         }
  132 
  133        134 135         //历史大戏留给关键人物
  136 
  137         mrCheng.start();
  138 
  139        
  140 
  141         try {
  142 
  143             //使其他线程等待当前线程执行完毕
  144 
  145             mrCheng.join();
  146 
  147         } catch (InterruptedException e) {
  148 
  149             e.printStackTrace();150 151         }
  152 
  153        
  154 
  155         System.out.println("战争结束，人民安居乐业，程先生实现了积极的人生梦想，为人民作出了贡献！");
  156 
  157         System.out.println("谢谢观看隋唐演义，再见！");
  158 
  159     }
  160 
  161    
  162 
  163     public static void main(String[] args) {
  164 
  165         new Stage().start();
  166 
  167     }
  168 
  169 }

Correct stopping of threads

How to correctly stop threads in Java:

Yes Control the exit of the loop through boolean type.

not stop method

The stop() method will stop the thread abruptly.

The stop() method is the wrong way to stop a thread.

Interaction of threads

Race Condition

When multiple threads share access to the same data (memory area) at the same time ), each thread attempts to operate on the data, causing the data to be corrupted. This phenomenon is called a race condition.

What is mutual exclusion?

How is mutual exclusion achieved?

Can only be accessed by one thread, mutually exclusive.

Mutual exclusion implementation: synchronized (intrinsic lock) lock.

Synchronization implementation: wait()/notify()/notifyAll().

How to expand your knowledge of Java concurrency

Java Memory Mode

JMM describes how Java threads interact through memory

Happens -before

            Synchronized, volatile&final

Locks&Condition

            High-level implementation of Java lock mechanism and wait conditions

           . ##Thread Safety

                                                                                                                                                                                                                                                    Atomicity and Visibility Thread-programming interactive model

# Producer-Consumer Model

Read-Write Lock Model

FUTURE Model

## Worker Thread Model Java5 complicate programming tools

java.util.concurrent

thread pool ExecutorService

Sering & FUTURE

## BLOCKINGQUEUE

## steam: Recommend two books:

Core Java

Java concurrency in practice

Comparison of two methods of thread creation

Thread:

① Inherit the Thread class;

Runnable:

① The Runnable method can avoid the defects of the Thread method due to the single inheritance feature of Java.

② Runnable code can be shared by multiple threads (Thread instances), which is suitable for situations where multiple threads process the same resource.

Case:

Thread:

 1 package com.czgo; 
 2  
 3   
 4  
 5 class MyThread extends Thread{ 
 6  
 7     
 8  
 9     private int ticketsCont = 5;    //一共有5张火车票
 10 
 11    
 12 
 13     private String name;            //窗口，也即是线程的名字
 14 
 15    
 16 
 17     public MyThread(String name){
 18 
 19         this.name = name;
 20 
 21     }
 22 
 23    24 25     @Override
 26 
 27     public void run() {
 28 
 29        30 31         while(ticketsCont>0){
 32 
 33             ticketsCont--;      //如果还有票，就卖掉一张
 34 
 35             System.out.println(name+"卖了1张票，剩余票数为："+ticketsCont);
 36 
 37         }
 38 
 39     }
 40 
 41    
 42 
 43 }
 44 
 45  
 46 
 47 public class TicketsThread {
 48 
 49  
 50 
 51     public static void main(String[] args) {
 52 
 53         //创建3个线程，模拟三个窗口卖票
 54 
 55         MyThread mt1 = new MyThread("窗口1");
 56 
 57         MyThread mt2 = new MyThread("窗口2");
 58 
 59         MyThread mt3 = new MyThread("窗口3");
 60 
 61        62 63         //启动这三个线程，也即是窗口，开始卖票
 64 
 65         mt1.start();
 66 
 67         mt2.start();
 68 
 69         mt3.start();
 70 
 71        
 72 
 73     }
 74 
 75  
 76 
 77 }

Runnable:

 1 package com.czgo; 
 2  
 3   
 4  
 5 class MyThread implements Runnable{ 
 6  
 7     
 8  
 9     private int ticketsCont = 5;    //一共有5张火车票
 10 
 11  
 12 
 13     @Override
 14 
 15     public void run() {
 16 
 17        
 18 
 19         while(ticketsCont>0){
 20 
 21             ticketsCont--;      //如果还有票，就卖掉一张
 22 
 23             System.out.println(Thread.currentThread().getName()+"卖了1张票，剩余票数为："+ticketsCont);
 24 
 25         }
 26 
 27        
 28 
 29     }
 30 
 31    
 32 
 33 }
 34 
 35  
 36 
 37 public class TicketsRunnable {
 38 
 39  
 40 
 41     public static void main(String[] args) {
 42 
 43        
 44 
 45         MyThread mt1 = new MyThread();
 46 
 47         MyThread mt2 = new MyThread();
 48 
 49         MyThread mt3 = new MyThread();
 50 
 51        
 52 
 53         //创建三个线程来模拟三个售票窗口
 54 
 55         Thread th1 = new Thread(mt1,"窗口1");
 56 
 57         Thread th2 = new Thread(mt2,"窗口2");
 58 
 59         Thread th3 = new Thread(mt3,"窗口3");
 60 
 61        
 62 
 63         //启动这三个线程，也即是三个窗口开始卖票
 64 
 65         th1.start();
 66 
 67         th2.start();
 68 
 69         th3.start();
 70 
 71  
 72 
 73     }
 74 
 75  
 76 
 77 }

Life cycle of thread

Illustration ：

Create: Create a new thread object, such as Thread thd = new Thread().

Ready: After creating the thread object, the thread's start() method is called (note: at this time, the thread just enters the thread queue, waiting to obtain cpu service, and has the conditions to run, but it does not necessarily have started running).

Run: Once the thread in the ready state obtains the CPU resource, it enters the running state and starts executing the logic in the run() method.

Termination: After the thread's run() method is executed, or the thread calls the stop() method, the thread enters the terminated state. Blocking: Under certain circumstances, an executing thread temporarily gives up CPU resources for some reason, suspends its own execution, and enters a blocking state, such as calling the sleep() method. .

The patron saint of threads-Guardian thread

There are two types of Java threads:

User Thread: runs in the foreground and performs specific tasks.

For example, the main thread of the program, the sub-threads connected to the network, etc. are all user threads.

Daemon thread: runs in the background and serves other foreground threads.

Features: Once all user threads have finished running, the daemon thread will end its work together with the JVM.

Application: Monitoring thread in the database connection pool, monitoring thread after the JVM virtual machine is started.

The most common daemon thread: garbage collection thread.

如何设置守护线程

可以通过调用Thread类的setDaemon(true)方法来设置当前线程为守护线程。

注意事项：

setDaemon(true)必须在start()方法之前调用，否则会抛出IllegalThreadStateException异常。

在守护线程中产生的新线程也是守护线程

不是所有的任务都可以分配给守护线程来执行，比如读写操作或者计算逻辑。

案例：

  1 package com.czgo;  
  2   
  3    
  4   
  5 import java.io.File;  
  6   
  7 import java.io.FileOutputStream;  
  8   
  9 import java.io.IOException; 
  10  
  11 import java.io.OutputStream; 
  12  
  13 import java.util.Scanner; 
  14  
  15   
  16  
  17 class DaemonThread implements Runnable{ 
  18  
  19     @Override 
  20  
  21     public void run() { 
  22  
  23         System.out.println("进入守护线程"+Thread.currentThread().getName()); 
  24  
  25         try { 
  26  
  27             writeToFile(); 
  28  
  29         } catch (IOException e) { 
  30  
  31             e.printStackTrace(); 
  32  
  33         } catch (InterruptedException e) { 34  35             e.printStackTrace(); 
  36  
  37         } 
  38  
  39         System.out.println("退出守护线程"+Thread.currentThread().getName()); 
  40  
  41         
  42  
  43     } 
  44  
  45     
  46  
  47     private void writeToFile() throws IOException, InterruptedException{ 
  48  
  49         File filename = new File("C:\\ide"+File.separator+"daemon.txt"); 
  50  
  51         OutputStream os = new FileOutputStream(filename,true); 
  52  
  53         int count = 0; 
  54  
  55         while(count<999){ 
  56  
  57             os.write(("\r\nword"+count).getBytes()); 
  58  
  59             System.out.println("守护线程"+Thread.currentThread().getName()+"向文件中写入了word"+count++); 
  60 
  61             Thread.sleep(1000); 
  62  
  63         } 
  64  
  65         os.close(); 
  66  
  67     } 
  68  
  69 } 
  70  
  71   
  72  
  73 public class DaemonThreadDemo { 
  74  
  75   
  76  
  77     public static void main(String[] args) { 
  78  
  79         
  80  
  81         System.out.println("进入主线程"+Thread.currentThread().getName()); 
  82  
  83         DaemonThread daemonThread = new DaemonThread(); 
  84  
  85         Thread thread = new Thread(daemonThread); 
  86  
  87         thread.setDaemon(true); 
  88  
  89         thread.start(); 
  90  
  91         
  92  
  93         Scanner sc = new Scanner(System.in); 
  94  
  95         sc.next();
  96  
  97         
  98  
  99         System.out.println("退出主线程"+Thread.currentThread().getName());
  100 
  101     }
  102 
  103  
  104 
  105 }

使用jstack生成线程快照

jstack

作用：生成jvm当前时刻线程的快照（threaddump，即当前进程中所有线程的信息）

目的：帮助定位程序问题出现的原因，如长时间停顿、cpu占用率过高等。

如何使用jstack

jstack –l pid

 

内存可见性

可见性介绍

可见性：一个线程对共享变量值的修改，能够及时地被其他线程看到。

共享变量：如果一个变量在多个线程的工作内存中都存在副本，那么这个变量就是这几个线程的共享变量。

Java内存模型（JMM）

Java内存模型(Java Memory Model)描述了程序中各种变量(线程共享变量)的访问规则，以及在Java中将变量存储到内存和从内存中读取出变量这样的底层细节。

所有变量都存储在主内存中

每个线程都有自己独立的工作内存，里面保存该线程使用到的变量的副本（主内存中该变量的一份拷贝）。

 

线程对共享变量的所有操作都必须在自己的工作内存中进行，不能直接从主内存中读写。

不同线程之间无法直接访问其他线程工作内存中的变量，线程间变量值的传递需要通过主内存来完成。

共享变量可见性实现的原理

线程1对共享变量的修改要想被线程2及时看到，必须要经过如下2个步骤：

把工作内存1中更新过的共享变量刷新到主内存中。

将主内存中最新的共享变量的值更新到工作内存2中。

可见性

要实现共享变量的可见性，必须保证两点：

线程修改后的共享变量值能够及时从工作内存刷新到主内存中。

其他线程能够及时把共享变量的最新值从主内存更新到自己的工作内存中。

可见性的实现方式

Java语言层面支持的可见性实现方式：

Synchronized

Volatile

Synchronized实现可见性

Synchronized能够实现：

原子性（同步）；

可见性

JMM关于Synchronized的两条规定：

线程解锁前，必须把共享变量的最新值刷新到主内存中；

线程加锁时，将清空工作内存中共享变量的值，从而使用共享变量时需要从主内存中重新读取最新的值（注意：加锁与解锁需要是同一把锁）

线程解锁前对共享变量得修改在下次加锁时对其他线程可见。

线程执行互斥代码的过程：

1. 获得互斥锁

2. 清空工作内存

3. 从主内存中拷贝变量的最新副本到工作内存

4. 执行代码。

5. 将更改后的共享变量的值刷新到主内存。

6. 释放互斥锁。

重排序

重排序：代码书写的顺序与实际执行的顺序不同，指令重排序是编译器或处理器为了提高程序性能而做的优化。

1. 编译器优化的重排序（编译器优化）。

2. 指令级并行重排序（处理器优化）。

3. 内存系统的重排序（处理器优化）。

as-if-serial

as-if-serial:无论如何重排序，程序执行的结果应该与代码顺序执行的结果一致(java编译器、运行时和处理器都会保证Java在单线程下遵循as-if-serial语义)。

例子：

Int num = 1;

Int num2 = 2;

Int sum = num+num2;

单线程：第1、2行的顺序可以重排，但第3行不能

重排序不会给单线程带来内存可见性问题

多线程中程序交错执行时，重排序可能会造成内存可见性问题。

Volatile实现可见性

Volatile关键字：

能够保证volatile变量的可见性

不能保证volatile变量复合操作的原子性

Volatile如何实现内存可见性：

In depth: It is achieved by adding memory barriers and prohibiting reordering optimization.

When a write operation is performed on a volatile variable, a store barrier instruction will be added after the write operation.

When a read operation is performed on a volatile variable, a load barrier instruction will be added before the read operation.

In layman's terms: Every time a volatile variable is accessed by a thread, the value of the variable is forced to be reread from the main memory. When the variable changes, the thread is forced to refresh the latest value to the main memory. In this way, different threads can always see the latest value of the variable at any time.

The process of thread writing volatile variables:

1. Change the value of the volatile variable copy in the thread's working memory

2. will be changed after The value of the copy is flushed from the working memory to the main memory

The process of thread reading volatile variables:

1. Reading volatile variables from main memory The latest value to the thread's working memory

2. Read a copy of the volatile variable from the working memory.

Volatile Applicable Occasions

To safely use volatile variables in multi-threads, you must also meet:

1. The writing operation to a variable does not depend on its current value

2. Boolean, used to record temperature

3. This variable is not included in an invariant with other variables.

Conclusion: Don’t care what others think about you behind your back, because these words can’t change the facts, but they may mess with your heart.

Category	Methods Signature	Introduction
Creation of thread	Thread()
	Thread(String name)
	Thread(Runnable target)
	Thread(Runnable target,String name)
Thread method	void start()	Start thread
	##static void sleep(long millis)	Thread sleep
	static void sleep(long millis,int nanos)
	void join()	Make other threads wait for the current thread to terminate
	void join( long millis)
	void join(long millis,int nanos)

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