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# Java - Thread Deadlock

A special type of error that you need to avoid that relates specifically to multitasking is deadlock, which occurs when two threads have a circular dependency on a pair of synchronized objects.

For example, suppose one thread enters the monitor on object X and another thread enters the monitor on object Y. If the thread in X tries to call any synchronized method on Y, it will block as expected. However, if the thread in Y, in turn, tries to call any synchronized method on X, the thread waits forever, because to access X, it would have to release its own lock on Y so that the first thread could complete.

## Example:

To understand deadlock fully, it is useful to see it in action. The next example creates two classes, A and B, with methods foo( ) and bar( ), respectively, which pause briefly before trying to call a method in the other class.

The main class, named Deadlock, creates an A and a B instance, and then starts a second thread to set up the deadlock condition. The foo( ) and bar( ) methods use sleep( ) as a way to force the deadlock condition to occur.

    class A {
       synchronized void foo(B b) {
          String name = Thread.currentThread().getName();
          System.out.println(name + " entered A.foo");
          try {
             Thread.sleep(1000);
          } catch(Exception e) {
             System.out.println("A Interrupted");
          }
          System.out.println(name + " trying to call B.last()");
          b.last();
       }
       synchronized void last() {
          System.out.println("Inside A.last");
       }
    }
    class B {
       synchronized void bar(A a) {
          String name = Thread.currentThread().getName();
          System.out.println(name + " entered B.bar");
          try {
             Thread.sleep(1000);
          } catch(Exception e) {
             System.out.println("B Interrupted");
          }
          System.out.println(name + " trying to call A.last()");
          a.last();
       }
       synchronized void last() {
          System.out.println("Inside A.last");
       }
    }
    class Deadlock implements Runnable {
       A a = new A();
       B b = new B();
       Deadlock() {
          Thread.currentThread().setName("MainThread");
          Thread t = new Thread(this, "RacingThread");
          t.start();
          a.foo(b); // get lock on a in this thread.
          System.out.println("Back in main thread");
       }
       public void run() {
          b.bar(a); // get lock on b in other thread.
          System.out.println("Back in other thread");
       }
       public static void main(String args[]) {
          new Deadlock();
       }
    }

Here is some output from this program:

    MainThread entered A.foo
    RacingThread entered B.bar
    MainThread trying to call B.last()
    RacingThread trying to call A.last()

Because the program has deadlocked, you need to press CTRL-C to end the program. You can see a full thread and monitor cache dump by pressing CTRL-BREAK on a PC .

You will see that RacingThread owns the monitor on b, while it is waiting for the monitor on a. At the same time, MainThread owns a and is waiting to get b. This program will never complete.

As this example illustrates, if your multithreaded program locks up occasionally, deadlock is one of the first conditions that you should check for.

# Ordering Locks:

Acommon threading trick to avoid the deadlock is to order the locks. By ordering the locks, it gives threads a specific order to obtain multiple locks.

## Deadlock Example:

Following is the depiction of a dead lock:

    // File Name ThreadSafeBankAccount.java
    public class ThreadSafeBankAccount
    {
       private double balance;
       private int number;
       public ThreadSafeBankAccount(int num, double initialBalance)
       {
          balance = initialBalance;
          number = num;
       }
       public int getNumber()
       {
          return number;
       }
       public double getBalance()
       {
          return balance;
       }
       public void deposit(double amount)
       {
          synchronized(this)
          {
            double prevBalance = balance;
            try
            {
               Thread.sleep(4000);
            }catch(InterruptedException e)
            {}
            balance = prevBalance + amount;
          }
       }
       public void withdraw(double amount)
       {
          synchronized(this)
          {
    	     double prevBalance = balance;
             try
             {
                Thread.sleep(4000);
             }catch(InterruptedException e)
             {}
             balance = prevBalance - amount;
          }
       }
    }

    // File Name LazyTeller.java
    public class LazyTeller extends Thread
    {
       private ThreadSafeBankAccount source, dest;
       public LazyTeller(ThreadSafeBankAccount a,
                         ThreadSafeBankAccount b)
       {
          source = a;
          dest = b;
       }
       public void run()
       {
          transfer(250.00);
       }
       public void transfer(double amount)
       {
          System.out.println("Transferring from "
              + source.getNumber() + " to " + dest.getNumber());
          synchronized(source)
          {
              Thread.yield();
              synchronized(dest)
              {
                 System.out.println("Withdrawing from "
                         + source.getNumber());
                 source.withdraw(amount);
                 System.out.println("Depositing into "
                         + dest.getNumber());
                 dest.deposit(amount);
              }
           }
       }
    }
    public class DeadlockDemo
    {
       public static void main(String [] args)
       {
          System.out.println("Creating two bank accounts...");
          ThreadSafeBankAccount checking =
                        new ThreadSafeBankAccount(101, 1000.00);
          ThreadSafeBankAccount savings =
                        new ThreadSafeBankAccount(102, 5000.00);

          System.out.println("Creating two teller threads...");
          Thread teller1 = new LazyTeller(checking, savings);
          Thread teller2 = new LazyTeller(savings, checking);
          System.out.println("Starting both threads...");
          teller1.start();
          teller2.start();
       }
    }

This would produce following result:

    Creating two bank accounts...
    Creating two teller threads...
    Starting both threads...
    Transferring from 101 to 102
    Transferring from 102 to 101

The problem with the LazyTeller class is that it does not consider the possibility of a race condition, a common occurrence in multithreaded programming.

After the two threads are started, teller1 grabs the checking lock and teller2 grabs the savings lock. When teller1 tries to obtain the savings lock, it is not available. Therefore, teller1 blocks until the savings lock becomes available. When the teller1 thread blocks, teller1 still has the checking lock and does not let it go.

Similarly, teller2 is waiting for the checking lock, so teller2 blocks but does not let go of the savings lock. This leads to one result: deadlock!

## Deadlock Solution Example:

Here transfer() method, in a class named OrderedTeller, in stead of arbitrarily synchronizing on locks, this transfer() method obtains locks in a specified order based on the number of the bank account.

    // File Name ThreadSafeBankAccount.java
    public class ThreadSafeBankAccount
    {
       private double balance;
       private int number;
       public ThreadSafeBankAccount(int num, double initialBalance)
       {
          balance = initialBalance;
          number = num;
       }
       public int getNumber()
       {
          return number;
       }
       public double getBalance()
       {
          return balance;
       }
       public void deposit(double amount)
       {
          synchronized(this)
          {
            double prevBalance = balance;
            try
            {
               Thread.sleep(4000);
            }catch(InterruptedException e)
            {}
            balance = prevBalance + amount;
          }
       }
       public void withdraw(double amount)
       {
          synchronized(this)
          {
    	     double prevBalance = balance;
             try
             {
                Thread.sleep(4000);
             }catch(InterruptedException e)
             {}
             balance = prevBalance - amount;
          }
       }
    }
    // File Name OrderedTeller.java
    public class OrderedTeller extends Thread
    {
       private ThreadSafeBankAccount source, dest;
       public OrderedTeller(ThreadSafeBankAccount a,
                            ThreadSafeBankAccount b)
       {
          source = a;
          dest = b;
       }
       public void run()
       {
          transfer(250.00);
       }
       public void transfer(double amount)
       {
           System.out.println("Transferring from " + source.getNumber()
               + " to " + dest.getNumber());
           ThreadSafeBankAccount first, second;
           if(source.getNumber() < dest.getNumber())
           {
              first = source;
              second = dest;
           }
           else
           {
              first = dest;
              second = source;
           }
           synchronized(first)
           {
              Thread.yield();
              synchronized(second)
              {
                 System.out.println("Withdrawing from "
                             + source.getNumber());
                 source.withdraw(amount);
                 System.out.println("Depositing into "
                             + dest.getNumber());
                 dest.deposit(amount);
              }
          }
       }
    }

    // File Name DeadlockDemo.java
    public class DeadlockDemo
    {
       public static void main(String [] args)
       {
          System.out.println("Creating two bank accounts...");
          ThreadSafeBankAccount checking =
                        new ThreadSafeBankAccount(101, 1000.00);
          ThreadSafeBankAccount savings =
                        new ThreadSafeBankAccount(102, 5000.00);

          System.out.println("Creating two teller threads...");
          Thread teller1 = new OrderedTeller(checking, savings);
          Thread teller2 = new OrderedTeller(savings, checking);
          System.out.println("Starting both threads...");
          teller1.start();
          teller2.start();
       }
    }

This would remove deadlock problem and would produce following result:

    Creating two bank accounts...
    Creating two teller threads...
    Starting both threads...
    Transferring from 101 to 102
    Transferring from 102 to 101
    Withdrawing from 101
    Depositing into 102
    Withdrawing from 102
    Depositing into 101

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