Thread overview

Almost all operating systems support running multiple tasks at the same time. A task is usually a program, and each running program is a process. When a program is running, it may contain multiple sequential execution flows, each of which is a thread.

Threads and processes

        Almost all operating systems support the concept of process. All running tasks usually correspond to a process. When a program enters memory for running, it becomes a process. A process is a program in the running process and has certain independent performance. A process is an independent unit for resource allocation and scheduling of the system. A process contains the following three characteristics.

1. Independence: a process is an independent entity in the system. It can have its own independent resources. Each process has its own private address space. Without the permission of the process itself, a user process cannot directly access the address space of other threads.
2. Dynamic: the difference between a process and a program is that a program is only a static set of instructions, while a process is an active set of instructions in the system. The concept of time is added to a process. A process has its own life cycle and various states. These concepts are not available in a program.

Concurrency: multiple processes can be executed concurrently on a single processor without mutual influence. Concurrency and parallelism are two concepts. Parallelism refers to the simultaneous execution of multiple instructions on multiple processors at the same time, and the simultaneous execution of only one instruction at the same time. However, multiple process instructions are executed in rapid rotation, which makes the macro system have The effect of multiple processes executing at the same time.

Advantages of multithreading

Threads are independent and concurrent in the program. Compared with the divided process, threads in the process have less isolation. They share memory, file handle and other application states of each process.

Because the partition size of threads is smaller than that of processes, the concurrency of multi threads is high. In the process of execution, processes have independent memory units, and multiple threads share memory, which greatly improves the running efficiency of the program. Threads have higher performance than processes. This is because threads in the same process have common features. Multiple threads share the same process virtual space. The environment shared by threads includes: process code block, process public data, etc. with these shared data, threads can easily communicate with each other. In summary, threads have the following points.

1. Memory cannot be shared between processes, but memory can be shared between threads.
2. When the system creates a process, it needs to reallocate system resources for the process, but the cost of creating a thread is much smaller, so it is more efficient to use multithreading to achieve multitasking concurrency than multiprocessing.

Thread creation starts

The steps to create a Thread class by inheriting the Thread class are as follows:

1. Define the subclass of the Thread class and re run() method of the class. The method body of the run() method represents the tasks that the Thread needs to complete. Therefore, the run() method is called the Thread execution body.
2. The Thread object is created by calling an instance of the Thread subclass.
3. Call the start() method of the thread object to start the thread.

//Create a Thread class by inheriting the Thread class
public class FirstThread extends Thread {

    private int i;

    // The method body of the run() method is the thread execution body
    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            // When the Thread class inherits the Thread, you can directly use this to get the current Thread
            // getName() of Thread object returns the name of the current Thread
            // So you can directly use the getName() method to return the name of the current thread

            System.out.println(getName() + "  " + i);


        }

    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            // Call the currentThread() method of Thread to get the current Thread
            System.out.println(Thread.currentThread().getName() + "   " + i);


            if (i == 20) {
                // Create first thread
                new FirstThread().start();
                // Create a second thread
                new FirstThread().start();

            }
        }
    }
}

Execution results

"D:\Program Files\Java\jdk1.8.0_151\bin\java" "-javaagent:D:\Program Files\JetBrains\IntelliJ IDEA 2017.1.4\lib\idea_rt.jar=55909:D:\Program Files\JetBrains\IntelliJ IDEA 2017.1.4\bin" -Dfile.encoding=UTF-8 -classpath "D:\Program Files\Java\jdk1.8.0_151\jre\lib\charsets.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\deploy.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\access-bridge-64.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\cldrdata.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\dnsns.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\jaccess.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\jfxrt.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\localedata.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\nashorn.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\sunec.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\sunjce_provider.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\sunmscapi.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\sunpkcs11.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\ext\zipfs.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\javaws.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\jce.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\jfr.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\jfxswt.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\jsse.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\management-agent.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\plugin.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\resources.jar;D:\Program Files\Java\jdk1.8.0_151\jre\lib\rt.jar;D:\github_project\Demo\javamaven\target\classes;E:\repository\org\yaml\snakeyaml\1.17\snakeyaml-1.17.jar;E:\repository\javax\validation\validation-api\1.1.0.Final\validation-api-1.1.0.Final.jar;E:\repository\com\fasterxml\jackson\core\jackson-databind\2.8.1\jackson-databind-2.8.1.jar;E:\repository\com\fasterxml\jackson\core\jackson-annotations\2.8.1\jackson-annotations-2.8.1.jar;E:\repository\com\fasterxml\jackson\core\jackson-core\2.8.1\jackson-core-2.8.1.jar;E:\repository\org\slf4j\slf4j-api\1.7.21\slf4j-api-1.7.21.jar" com.thread.FirstThread
main   0
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Thread-0  0
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Process finished with exit code 0

The FirstThread class of the above program inherits the Thread class and implements the Run() method, as shown in the first bold code in the program. The code execution flow in the Run() method is the task that the Thread needs to complete. The main method of the program also contains a loop. When the loop variable i is equal to 20, two new threads are created and started, Running the above program, although the above program only shows the creation and startup of two threads, there are actually three threads in the program, that is, the program explicitly creates two sub threads and one main Thread. When the Java program starts to run, the program will create at least one main Thread, and the Thread execution body of the main Thread is not determined by the Run() method, but by the main() method, mai The body of the n () method represents the Thread body of the main Thread.