Deadlock describes a
situation where two or more threads are blocked forever, waiting for each
other.
Deadlock occurs because:
Mutual exclusion - Two processes cannot simultaneously control the same resource or be in their critical section.
Hold
and Wait - processes currently holding resources can request new
resources.
No
preemption - Once a process holds a resource, it cannot
be taken away by another process or the kernel.
Circular
wait - Each process is waiting to obtain a resource which is
held by another process.
Deadlocks in Java:
Deadlocks can occur in
Java when the synchronized keyword causes the executing thread to block while
waiting to get the lock, associated with the specified object. Since the thread
might already hold locks associated with other objects, two threads could each
be waiting for the other to release a lock. In such case, they will end up
waiting forever.
Deadlock Example in java
class Thread1 implements Runnable {
Deadlock Example in java
public Object obj1;
public Object obj2;
public Thread1(Object obj1, Object
obj2) {
this.obj1 = obj1;
this.obj2 = obj2;
}
public void run() {
while(true) {
synchronized (obj1) {
System.out.println("Thread1 : obj1");
synchronized (obj2) {
System.out.println("Thread1 : obj2");
}
}
}
}
}
class Thread2 implements Runnable {
public Object obj1;
public Object obj2;
public Thread2(Object obj1, Object
obj2) {
this.obj1 = obj1;
this.obj2 = obj2;
}
public void run() {
while(true) {
synchronized (obj2) {
System.out.println("Thread2 : obj2");
synchronized (obj1) {
System.out.println("Thread2 : obj1");
}
}
}
}
}
public class DeadLock {
public static void main(String args[]) throws InterruptedException {
Object
obj1 = new Object();
Object
obj2 = new Object();
Thread2
runn2 = new
Thread2(obj1, obj2);
Thread1
runn1 = new
Thread1(obj1, obj2);
Thread
thrd2 = new
Thread(runn2);
Thread
thrd1 = new
Thread(runn1);
thrd1.start();
thrd2.start();
while(!thrd2.isAlive() ||
!thrd1.isAlive()) {
System.out.println("No deadlock found");
}
}
}
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