Deadlock and multithreaded communication problems

deadlock

Example

public static void main(String[] args) { Criminal c = new Criminal(); Police p = new Police(); Mythread thread = new Mythread(c,p); thread.start();// Police shout c.say(p);// Criminals shout } static class Mythread extends Thread{ private Criminal c; private Police p; public Mythread(Criminal c,Police p){// Make sure you are the same person as the police criminals in the main method this.c =c; this.p = p; } @Override public void run() { p.say(c); // Police shout } } static class Criminal{// Criminals public synchronized void say(Police p){// The criminal shouted to the police System.out.println("If you let me go, I'll take the hostage"); p.fun(); // Get a response from the police to release the criminal } public synchronized void fun(){// Response from the criminal System.out.println("Hostage Release,Criminals Escape"); } } static class Police{ public synchronized void say(Criminal c){// The police shouted at the prisoner System.out.println("If you take a hostage, I'll let you go"); c.fun(); // Get a response to the prisoner's release of the hostage } public synchronized void fun(){// Police response System.out.println("Rescue succeeded,Criminals Escape"); } } }

One of the possible outcomes (deadlock)

Analysis

• In this program, there are two threads, one is the prisoner's shout in the main thread, the other is the police's shout in Mythread s, where the police criminals are all the same object
• After the criminal shouts, he will lock the criminal object. Then he calls the response from the police object. After the police shout, he will also lock his own police object and wait for the criminal's response. At this time, both objects are locked and waiting for the other party's response, which will lead to a deadlock.

Solve

Do not call another locked method in one locked method

Multithreaded communication problem

Some resources are shared by some threads. When a thread is using these resources, another thread that needs resources should wait using the wait() method until the first thread uses the resources and wakes up the waiting threads using the notify() method to achieve resource scheduling.

Example

public static void main(String[] args) { Food f = new Food(); new Cook(f).start(); new Waiter(f).start(); } static class Cook extends Thread{// Chefs private Food f; public Cook(Food f){// Get food objects this.f=f; } @Override public void run() { try { for(int i = 0 ;i< 10;i++){//Make 10 dishes of steamed dumplings and mixed noodles if(i%2==0){ f.setNameAndPrice("Steamed dumpling","7 element"); }else { f.setNameAndPrice("Noodles","5 element"); } } } catch (InterruptedException e) { e.printStackTrace(); } } } static class Waiter extends Thread{// Server class private Food f; public Waiter(Food f) { this.f = f; } @Override public void run() { for(int i=0;i<10;i++){// Take away the cook's dishes try { Thread.sleep(100);// Avoid end too fast } catch (InterruptedException e) { e.printStackTrace(); } f.get(); } } } static class Food{ String name; String price; public void setNameAndPrice( String name,String price) throws InterruptedException { // Chef production process: label-rest 0.1 seconds-label price this.name= name; Thread.sleep(100);//Adding sleep makes problems easier this.price=price; } public void get(){// Server take away System.out.println(this.name+this.price); } }

Result

Analysis

Solve

Modify the set and get methods with synchronized and wake up the waiter at the end of the set method to let the chef sleep; wake up the chef at the end of the get method to let the waiter sleep

package xzk.chapter4.section5; public class Eat { public static void main(String[] args) { Food f = new Food(); new Cook(f).start(); new Waiter(f).start(); } static class Cook extends Thread{// Chefs private Food f; public Cook(Food f){// Chef's food object this.f=f; } @Override public void run() { try { for(int i = 0 ;i< 10;i++){// Make 10 portions of food, 5 steamed dumplings and 5 noodles if(i%2==0){ f.setNameAndPrice("Steamed dumpling","7 element"); } else { f.setNameAndPrice("Noodles","5 element"); } } } catch (InterruptedException e) { e.printStackTrace(); } } } static class Waiter extends Thread{// Server class private Food f; public Waiter(Food f) { this.f = f; } @Override public void run() { for(int i=0;i<10;i++){// End course (name and price output) try { Thread.sleep(100);// Avoid end too fast } catch (InterruptedException e) { e.printStackTrace(); } try { f.get(); } catch (InterruptedException e) { e.printStackTrace(); } } System.out.println("End"); this.interrupt(); } } static class Food{// Food String name; String price; private int count =0; public synchronized void setNameAndPrice( String name,String price) throws InterruptedException { // Food preparation methods this.name = name; Thread.sleep(100);//Joining sleep makes communication problems easier this.price = price; count++; this.notifyAll(); this.wait(); } public synchronized void get() throws InterruptedException {// Serving method System.out.println(this.name + this.price); count++; this.notifyAll(); //this.wait() //Using a parameterless wait method will keep the server process dormant after the last call to the get method this.wait(500);// A wait of 0.5 seconds should be used, so that neither the program nor an infinite wait occurs } } }