CODEWITHSUNDEEP
javadeadlockconcurrency
l0r3nz4cc10
l0r3nz4cc10

Reputation: 1283

Java synchronized vs deadlock example

I am not familiar with threads and concurrent programming. I was looking for a simple snippet which would result in a deadlock, here it is :

public class TestLock {
    private static class fun {
        int a,b;  

        void read() {System.out.println(a+b);}
        void write(int a,int b) {this.a=a;this.b=b;}
    }

    public static void main (String[] args) throws java.lang.Exception {
        final  fun d1=new fun();  
        final  fun d2=new fun();  

        Thread t1=new Thread() {  
            public void run() {  
                for(int i=0;i<5;i++) {
                    synchronized(d2) {
                        d2.read();  
                        try {  
                            Thread.sleep(50);  
                        } catch (Exception ex) {  
                            ex.printStackTrace();  
                        }  
                        synchronized(d1) {  
                            d1.write(i, i);  
                    }
                }  
            }  
        };

        Thread t2=new Thread() {  
            public void run() {  
                for(int i=0;i<5;i++) {  
                    synchronized(d1) {  
                        d1.read();  
                        try {  
                            Thread.sleep(50);  
                        } catch (Exception ex) {  
                            ex.printStackTrace();  
                        }  
                        synchronized(d2) {  
                            d2.write(i, i);  
                        }
                    }
                }
            }
        };  
        t1.start();  
        t2.start();  
    }
}

Now I was wondering how I could transform this example, using ReentrantLock instead of synchronized, but I don't see how : does fun need to have a ReentrantLock attribute in order to have something like

Thread t1=new Thread() {  
    public void run() {  
        for(int i=0;i<5;i++) {
            if(d2.lock.tryLock()) {
                    try {d1.read();Thread.sleep(50);} catch(Exception e) {e.printStackTrace();} finally {d1.lock.unlock();}
                        if(d2.lock.tryLock()) {
                            try {d2.write(i, i);} catch(Exception e) {e.printStackTrace();} finally {d2.lock.unlock();}
                        }
                    }
                }  
            }  
        };

or am I missing something entirely ?

Upvotes: 1

Views: 205

Answers (1)

JB Nizet
JB Nizet

Reputation: 691715

Transforming the example using ReentrantLocks would indeed mean using two locks: one associated with d1 and the other one associated with d2.

And you would replace every entrance in a synchronized block on dX by a call to lockX.lock(), and any exit from a synchronized block on dX by a call to lockX.unlock()`.

Using tryLock() defeats the purpose, since it returns instead of waiting if the lock can't be acquired.

Upvotes: 1

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