How to Solve Deadlock using Threads in Java?

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If two threads are waiting for each other forever such type of infinite waiting is called deadlock in java. Synchronized keyword is the only reason for deadlock situation hence while using synchronized keyword we have to take special care. There is no resolution technique for deadlock, but several prevention techniques are available.

Implementation: Deadlock occurs

Example 1:

Java

// Java program to illustrate Deadlock
// where deadlock occurs
 
// Importing required packages
import java.io.*;
import java.util.*;
 
// Class 1
// Helper class
class A {
 
    // Method 1 of this class
    // Synchronized method
    public synchronized void last()
    {
 
        // Print and display statement
        System.out.println("Inside A, last() method");
    }
 
    // Method 2 of this class
    // Synchronized method
    public synchronized void d1(B b)
    {
        System.out.println(
            "Thread1 start execution of d1() method");
 
        // Try block to check for exceptions
        try {
 
            // Putting the current thread to sleep for
            // specific time using sleep() method
            Thread.sleep(2000);
        }
 
        // Catch block to handle the exceptions
        catch (InterruptedException e) {
            // Display the exception on the console
            System.out.println(e);
        }
 
        // Display statement
        System.out.println(
            "Thread trying to call B's last() method");
 
        // Calling the method 1 of this class as created
        // above
        b.last();
    }
}
 
// Class 2
// Helper class B
class B {
 
    // Method 1 of this class
    public synchronized void last()
    {
 
        // Display statement only
        System.out.println("Inside B, last() method");
    }
 
    // Method 2 of this class
    // Synchronized the method d2
    public synchronized void d2(A a)
    {
 
        // Display message only
        System.out.println(
            "Thread2 start execution of d2() method");
 
        // Try block to check for exceptions
        try {
 
            // Putting the current thread to sleep for
            // certain time using sleep() method
            Thread.sleep(2000);
 
            // Catch block to handle the exceptions
        }
        catch (InterruptedException e) {
 
            // Display the exception on the console
            System.out.println(e);
        }
 
        // Display message only
        System.out.println(
            "Thread2  trying to call A's last method");
 
        // Again calling the last() method inside this class
        a.last();
    }
}
 
// Class 3
// Main class
// Deadlock class which is extending Thread class
class GFG extends Thread {
 
    // Creating object of type class A
    A a = new A();
 
    // Creating object of type class B
    B b = new B();
 
    // Method 1
    public void m1()
    {
 
        // Starting the thread
        this.start();
 
        // Calling d1 method of class A
        a.d1(b);
    }
 
    // Method 2
    // run() method for the thread
    public void run()
    {
 
        // Calling d2 method of class B
        b.d2(a);
    }
 
    // Method 3
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating object of this class
        GFG deadlock = new GFG();
 
        // Calling m1 method
        deadlock.m1();
    }
}

Output:

Output explanation:

Here the cursor is showing forever because the threads enter into the deadlock situation. In the above program if we removed at least one synchronized keyword then the program won’t enter into the deadlock situation. Hence, synchronized keyword is one of the major reason for deadlock situation. Due to this while using synchronized keyword we have to take special care.

We can avoid Deadlock situation in the following ways:

Using Thread.join() Method: We can get a deadlock if two threads are waiting for each other to finish indefinitely using thread join. Then our thread has to wait for another thread to finish, it is always best to use Thread.join() method with the maximum time you want to wait for the thread to finish.
Use Lock Ordering: We have to always assign a numeric value to each lock and before acquiring the lock with a higher numeric value we have to acquire the locks with a lower numeric value.
Avoiding unnecessary Locks: We should use locks only for those members on which it is required, unnecessary use of locks leads to a deadlock situation. And it is recommended to use a lock-free data structure and If it is possible to keep your code free from locks. For example, instead of using synchronized ArrayList use the ConcurrentLinkedQueue.

Example 2: Deadlock is prevented

Java

// Java program to illustrate Deadlock
// where deadlock is prevented from occurring
 
// Importing required packages
import java.io.*;
import java.util.*;
 
// Class 1
// Helper class
class A {
 
    // Method 1 of this class
    // Synchronized method
    public synchronized void last()
    {
 
        // Print and display statement
        System.out.println("Inside A, last() method");
    }
 
    // Method 2 of this class
    // Synchronized method
    public synchronized void d1(B b)
    {
        System.out.println(
            "Thread1 start execution of d1() method");
 
        // Try block to check for exceptions
        try {
 
            // Putting the current thread to sleep for
            // specific time using sleep() method
            Thread.sleep(2000);
        }
 
        // Catch block to handle the exceptions
        catch (InterruptedException e) {
            // Display the exception on the console
            System.out.println(e);
        }
 
        // Display statement
        System.out.println(
            "Thread trying to call B's last() method");
 
        // Calling the method 1 of this class as created
        // above
        b.last();
    }
}
 
// Class 2
// Helper class B
class B {
 
    // Method 1 of this class
    public void last()
    {
 
        // Display statement only
        System.out.println("Inside B, last() method");
    }
 
    // Method 2 of this class
    // Non-synchronized the method d2
    public void d2(A a)
    {
 
        // Display message only
        System.out.println(
            "Thread2 start execution of d2() method");
 
        // Try block to check for exceptions
        try {
 
            // Putting the current thread to sleep for
            // certain time using sleep() method
            Thread.sleep(2000);
 
            // Catch block to handle the exceptions
        }
        catch (InterruptedException e) {
 
            // Display the exception on the console
            System.out.println(e);
        }
 
        // Display message only
        System.out.println(
            "Thread2  trying to call A's last method");
 
        // Again calling the last() method inside this class
        a.last();
    }
}
 
// Class 3
// Main class
// Deadlock class which is extending Thread class
class GFG extends Thread {
 
    // Creating object of type class A
    A a = new A();
 
    // Creating object of type class B
    B b = new B();
 
    // Method 1
    public void m1()
    {
 
        // Starting the thread
        this.start();
 
        // Calling d1 method of class A
        a.d1(b);
    }
 
    // Method 2
    // run() method for the thread
    public void run()
    {
 
        // Calling d2 method of class B
        b.d2(a);
    }
 
    // Method 3
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating object of this class
        GFG deadlock = new GFG();
 
        // Calling m1 method
        deadlock.m1();
    }
}