Multithreading in Java

In Java , implementing multithreading requires the use of the Thread class .

Thread class definition

public class Thread extends Object implements Runnable

   

void	run() If the thread was constructed using a standalone Runnable run object, the run method of the Runnable object is called; otherwise, the method does nothing and returns.
void	start() Causes the thread to start executing; the Java virtual machine calls the thread's run method.

   

Implement multithreading method 1:

The custom class inherits the Thread class .

Override run () method in custom class .

Create object .

start thread

   

   

   

There are two scheduling models for threads

time-sharing scheduling model

All threads use the CPU usage rights in turn, and the CPU time slice occupied by each thread is evenly distributed .

Preemptive scheduling model

Threads with higher priorities are given priority to use the CPU. If the priorities of the threads are the same , one will be randomly selected , and the threads with higher priorities will obtain relatively more CPU time slices .

Java uses a preemptive scheduling model .

   

Methods of the Thread class

Thread priority setting

java.lang.Thread
public static final int	MAX_PRIORITY	10
public static final int	MIN_PRIORITY	1
public static final int	NORM_PRIORITY	5

   

Thread sleep settings

static void

sleep(long millis) Sleeps (suspends execution) the currently executing thread for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers.

   

   

comity of thread

static void

yield() Pause the currently executing thread object and execute other threads.

   

Thread politeness can make thread allocation more balanced in a certain program .

   

Daemon thread

   

void	setDaemon(boolean on) Mark the thread as a daemon thread or a user thread.

   

For programs marked as daemon threads , the daemon threads are also launched after the main thread exits .

   

thread terminated

public final void stop() Force the thread to stop executing.

The stop method is officially not recommended , stop will directly stop the thread and will not continue to execute downwards. It has security risks.

   

   

kill thread

void	interrupt() interrupt the thread.

   

Abort the state of the thread and throw an exception .

   

   

Implement multi-threading method 2

Custom class implements Runable interface

Override run method in custom class

Create objects of custom classes .

Create an object of the Thread class and pass the object of the custom class as a constructor parameter .

   

   

   

Practice questions , simulate the ticketing program , assuming a total of 100 tickets , there are three windows selling tickets at the same time .

Method 1, the method of inheriting the Thread class

   

   

Mode 2, the way to implement the Runable interface .

   

   

In order to simulate the real situation , set a delay of 100ms in the program .

   

As can be seen from the results , there are cases where a ticket is sold twice and negative tickets are sold

   

   

Are there any issues that can cause problems ?

Whether the multi-threaded environment

Is there any shared data

Whether multiple statements operate on shared data

   

This problem can be solved by using synchronize d . The code contained in the synchronized can only be executed by one object at any time.

   

The synchronize d keyword can also modify methods

   

Lock object for synchronized code blocks

any object

Synchronized method format and lock object

Add the synchronization keyword to the method .

What is the object of the synchronized method ?

this, that is to say, if the same code needs to be locked outside the method, the synchronization object this is required.

Static method and lock object problem

The lock object of the static method is the bytecode file object of the class . ( eg SellTicket.class)

   

Methods for creating thread-safe collection classes