Multithreading basics notes

One 、 Threads

1. Basic concepts

  • Program (program): To accomplish a specific task 、 A collection of instructions written in a language . That is to say A piece of static code , Static objects .
  • process (process): It's an execution of the program , or A running program . It's a dynamic process : It has its own production 、 The process of existence and extinction ( Life cycle ).
    • Process as the unit of resource allocation , At run time, the system allocates different memory areas for each process .
  • Threads (tread): Processes can be further refined into threads , It's an execution path inside a program .
    • If a process executes multiple threads at the same time , It supports multithreading .
    • Thread as the unit of scheduling and execution , Each thread has its own Run stack and Program counter (pc), Low cost of thread switching .
    • Multiple threads in a process share the same memory unit / Memory address space --> They allocate objects from the same heap , You can access the same variables and objects . This makes communication between threads easier 、 Efficient . But the system resources shared by multiple threads may bring security risks .
  • Single core CPU And polynuclear CPU The understanding of the :
    • Single core CPU, It's actually a fake multithreading , Because in a unit of time , Only one thread can be executed . for example : There are multiple lanes , But there is only one staff member in the toll station charging , You can only pass if you pay a fee , that CPU It's like a toll collector , If someone doesn't want to pay , Then the toll collector can take him “ Hang up “( Wait until he's ready to pay ). But because CPU The time unit is very short , So I can't feel it .
    • Multicore CPU, In order to better play the efficiency of multithreading .
    • One Java Applications java.exe, There are at least three threads :main() The main thread , gc() Garbage collection thread , Exception handling threads . Of course, if something goes wrong , Will affect the main thread .
  • Parallelism and concurrency
    • parallel : Multiple CPU Perform multiple tasks at the same time .
    • Concurrent : One CPU( Using time slices ) Perform multiple tasks at the same time .

2. Advantages of multithreading

  1. Improve application response . More meaningful for graphical interfaces , Enhance the user experience .
  2. Improving computer systems CPU Utilization ratio .
  3. Improve program structure . Divide a long and complex process into multiple threads . Independent operation , Easy to understand and modify .

3. When multithreading is needed

  1. Programs need to perform two or more tasks at the same time .
  2. When the program needs to implement some tasks that need to wait , Such as user input 、 File read and write 、 Network operating 、 Search, etc .
  3. When you need some background programs .

4. Thread creation

Mode one : Inherited from Thread class

  1. Create an inheritance from Thread Subclasses of classes
  2. rewrite Thread Class run Method --> Declare the operation performed by this thread in run() in
  3. establish Thread Object of subclass of class
  4. Call... Through objects start Method

Example :

//1. Create an inheritance from Thread Subclasses of classes 
class MyThread extends Thread {
//2. rewrite Thread Class run Method --> Declare the operation performed by this thread in run() in
public void run() {
// The function of this thread
} public class ThreadTest {
public static void main(String[] args) {
//3. establish Thread Object of subclass of class
MyThread t1 = new MyThread();
//4. Call... Through objects start Method
  • Be careful :

    • You can't call... Directly run() Start the thread in the same way ( It's essentially just a method call , Single thread )
    • Do not let the thread being started execute any code ( Report errors IllegalThreadException). You need to recreate a new thread object .

Mode two : Realization Runnable Interface

  1. Create an implementation Runnable The class of the interface

  2. Implement classes to implement Runnable Abstract methods in :run()

  3. Create objects that implement classes

  4. Pass this object as an argument to Thread In the constructor of class , establish Thread Class object

  5. adopt Thread Class object call start()

    ( It's actually calling theta Runnable Type of target)

    Example :

    //1, Create an implementation Runnable The class of the interface 
    class Mthread implements Runnable {
    public void run() {
    //2. rewrite Runnable Abstract methods in run()
    } public class ThreadTest1 {
    public static void main(String[] args) {
    //3. Create objects that implement classes
    MThread mThread = new MThread();
    //4. Pass the object as a parameter to Thread In the constructor of class , establish Thread Class object
    Thread t1 = new Thread(mThread);
    //5, adopt Thread Class object call start()

A comparison of the two ways

  1. Priority in development : Realization Runnable How to interface

    reason :

    1. The way of implementation is not limited by the single inheritance of class
    2. The implementation method is more suitable to deal with the situation that multiple threads have shared data
  2. Actually Thread The class itself inherits Runnable Interface , And built in the rewritten run().

Two 、Thread Class

  • void start() : Start thread , And execute the object's run() Method
  • run() : It usually needs to be rewritten Thread Class , Declare the operation to be performed by the created thread in this method
  • String getName() : Returns the name of the thread
  • void setName(String name) : Set the thread name
  • static Thread.currentThread() : Returns the current thread . stay Thread In subclasses this, It is usually used for the main thread and Runnable Implementation class
  • yield() : Release the current cpu The enforcement of
  • join() : In a thread a Calling thread b The thread of join(), The thread a Go into blocking mode , Until the thread b After full implementation , Threads a To end the blocking .
  • stop() : Deprecated . Force end of current thread .
  • sleep(long millitime) : Make the current thread “ sleep ” Fixed value millitime millisecond . At the designated millitime In milliseconds , The current thread is blocked .
  • isAlive() : Determine whether the current thread is alive ( The return value is boolean)

3、 ... and 、 Scheduling of threads

1. Dispatch

  • Scheduling strategy

    • Time slice :
    • Preemptive : High priority thread preemption CPU
  • Java The scheduling method of
    • Threads of the same priority form a first in first out queue ( First come, first serve ), Using time slice strategy
    • For high priority , Preemptive strategy using priority scheduling

2. Thread priority

  • The default configuration :

  • Set operation

    • Thread object name .getPriority()

      Get thread priority

    • Thread object name .setPriority()

      Set thread priority

  • Priority understanding

    High priority threads should preempt low priority threads cpu The enforcement of . But just in terms of probability , High priority threads are executed with high probability . It doesn't mean that only when the high priority thread finishes executing , Low priority threads only execute

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