Writing a deadlock and resolving in java

In java, a deadlock is a situation where minimum two threads are holding lock on some different resource, and both are waiting for other’s resource to complete its task. And, none is able to leave the lock on resource it is holding. (See image below)

In above case, Thread- has A but need B to complete processing and Similarly Thread-2 has resource B but need A first.

Let write above scenario in java code:

package thread;  
public class ResolveDeadLockTest {       
public static void main(String[] args) {         
ResolveDeadLockTest test = new ResolveDeadLockTest();           
final A a = test.new A();         
final B b = test.new B();           
// Thread-1         
Runnable block1 = new Runnable() {             
public void run() {                 
synchronized (a) {                     
try {                         
// Adding delay so that both threads can start trying to                         
// lock resources                         
Thread.sleep(100);                     
} catch (InterruptedException e) {                         
e.printStackTrace();                     
}                     
// Thread-1 have A but need B also                     
synchronized (b) {                         
System.out.println("In block 1");                     
}                 
}             
}         
};           
// Thread-2         
Runnable block2 = new Runnable() {             
public void run() {                 
synchronized (b) {                     
// Thread-2 have B but need A also                     
synchronized (a) {                         
System.out.println("In block 2");                     
}                 
}             
}         
};           
new Thread(block1).start();         
new Thread(block2).start();     
}       
// Resource A     
private class A {         
private int i = 10;           
public int getI() {             
return i;         
}           
public void setI(int i) {             
this.i = i;         
}     
}       
// Resource B     
private class B {         
private int i = 20;           
public int getI() {             
return i;         
}           
public void setI(int i) {             
this.i = i;         
}     
} 
} 

Running above code will result in deadlock for very obvious reasons (explained above). Now we have to solve this issue.

I believe, solution to any problem lies in identifying the root of problem. In our case, it is the pattern of accessing A and B, is main issue. So, to solve it, we will simply re-order the statements where code is accessing shared resources.

After rewriting the code, it will look like this ::

// Thread-1 
Runnable block1 = new Runnable() {     
public void run() {         
synchronized (b) {             
try {                 
// Adding delay so that both threads can start trying to                 
// lock resources                 
Thread.sleep(100);             
} catch (InterruptedException e) {                 
e.printStackTrace();             
}             
// Thread-1 have A but need B also             
synchronized (a) {                 
System.out.println("In block 1");             
}         
}     
} 
}; 
// Thread-2 
Runnable block2 = new Runnable() {     
public void run() {         
synchronized (b) {             
// Thread-2 have B but need A also             
synchronized (a) {                 
System.out.println("In block 2");             
}         
}     
} 
}; 

Run again above class, and you will not see any deadlock kind of situation. I hope, it will help you in avoiding deadlocks, and if encountered, in resolving them.

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