A deadlock is a common problem in concurrent programming, where two or more threads are blocked indefinitely, waiting for each other to release a resource. This situation can arise when two threads have a circular dependency on each other and both are waiting for the other to release a resource.
In Java, a deadlock can occur when two threads are each holding a lock on a resource and waiting to acquire a lock on the other resource. For example, if Thread A holds a lock on resource X and is waiting for a lock on resource Y, while Thread B holds a lock on resource Y and is waiting for a lock on resource X, a deadlock will occur.
Deadlocks can be difficult to diagnose and resolve because they can occur unpredictably, and their symptoms can be misleading. For example, if a deadlock occurs, one or both of the threads involved may appear to be stuck or blocked, even though the JVM is still running.
Example:-
public class DeadLockEx1 {
public static void main(String[] args) {
String s1 = "Hello";
Integer i =5;
Thread t1 = new Thread()
{
public void run()
{
synchronized(s1)
{
System.out.println("thread 1 : locked "+s1);
try
{
Thread.sleep(1000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
synchronized (i)
{
System.out.println("thread 1 : locked i");
}
}
}
};
Thread t2 = new Thread()
{
public void run()
{
synchronized(i)
{
System.out.println("thread 2 : locked "+i);
try
{
Thread.sleep(1000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
synchronized(s1)
{
System.out.println("thread 2 : locked s1");
}
}
}
};
t1.start();
t2.start();
}
}
To prevent deadlocks, it's important to follow best practices in concurrent programming, such as using a lock ordering policy, avoiding circular dependencies between locks, and releasing locks as soon as possible.
In addition, it's a good idea to monitor your application for deadlocks and implement a deadlock detection and recovery mechanism. This can be done using the java.lang.management API, which provides information about the state of the JVM, including any deadlocks that may have occurred.
In conclusion, deadlocks are a common problem in concurrent programming, but can be prevented and resolved by following best practices and monitoring your application for deadlocks. By taking steps to detect and resolve deadlocks, you can ensure the stability and reliability of your Java applications.
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