Understand in depth why not use system.out.println()

One 、 Preface

About why not use... In the project System.out.println() The explanation of , There are a lot of information on the Internet, but it is not accurate or detailed , Today, we will make an in-depth analysis of its causes and possible impact on the project .

The atmosphere of this article is three The main points of , The source code parsing 、 The impact analysis 、 Code demonstration .

Two 、 The source code parsing

open println Source code , notice println The method body contains synchronized, That is, the control with lock , In other words, there will be the possibility of blocking , Later we will think of creating congestion to demonstrate .

Here's the picture ：

open out Source code , Find out out use final static To modify , That's static variables , It can be considered as a single machine jvm This variable is globally unique in , Here's the picture ：

 

3、 ... and 、 The impact analysis

Through the above two figures , We can think of such a scene , stay System.out.println() In operation , Or for system reasons or other reasons , Leading to the execution of println When , There's a blockage , So the consequence is , All use System.out.println() The place of , Will be in block state , Even lead to system downtime and so on .

as for System.out.println() Its own performance problem , It can be used as a simple performance optimization , It is also not recommended to use , Since it is an in-depth analysis , Let's look at the serious scene first , This is what leads to disabling Syste.out.println() Important reasons .

Four 、 Code emulation

According to the source code , Simulate such a scenario , For some reason ,System.out In the locked state , It hasn't been released for a long time , What will happen .

Simulation steps ：

1、 Create two new threads ： Thread one and thread two

2、 Let thread one and thread two run for three seconds , Began to System.out Lock in

3、 Check that thread one and thread two are in System.out Printing and thread status when locking

4、 Release System.out lock , Check the operation of thread 1 and thread 2 here

Here is the test code ：

 

package com.zhufeng.thread;


public class ZhufengDemo {

    public static Object o = new Object();

    public static void main(String[] args) throws InterruptedException {
        ZhufengThread t1= new ZhufengThread(" One ");
        ZhufengThread t2= new ZhufengThread(" Two ");

        Thread thread1 = new Thread(t1);
        Thread thread2 = new Thread(t2);

        thread1.start();
        thread2.start();


        System.out.println("3 Seconds later, right  System.out  Lock ...");
        Thread.sleep(3000);
        //synchronized ( o){
        synchronized ( System.out){
            System.out.println(" Locked ...");
            for(int i=10;i>0;i--){
                System.out.println(" count down :"+i+"—— Thread status :"
                        +thread1.getState()+", Thread 2 status "+thread2.getState());
                Thread.sleep(1000);
            }
        }
        System.out.println(" Lock released ...");
    }
}

The operation logic here is carried out according to the steps described just now , Only... Is used in the thread System.out.println Print , stay System.out When locked , See if the two threads will continue printing

package com.zhufeng.thread;


public class ZhufengThread implements Runnable {

    private String name;
    public ZhufengThread(String name){
        this.name=name;
    }
    @Override
    public void run() {
        while (true){
            System.out.println(name+" Thread running ...");
            try {
                Thread.sleep(1500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

Every in the thread 1.5 Print once per second .

We run the print program , Check the console log ：

Conclusion ：

You can see through the console log , Thread one and thread two are running , If System.out When locked , Two threads are in BLOCKED state , And not printing , wait for 10 second System.out When the lock is released , Start printing again .

So , If it is widely used in production System.out.println(), It will block all logic that uses this statement , The impact is self-evident .