Synchronization in Java

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An important consideration when designing multi-threaded applications is conflict over access to data If two threads are fighting for the same resource, and a mechanism to resolve access conflicts is not put into place, the integrity of the application is at stake Built into the Java language are two mechanisms for preventing concurrent access to resources: method-level synchronization and block-level synchronization
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731 Method-Level Synchronization
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Method-level synchronization prevents two threads from executing methods on an object at the same time Methods that must be "thread-safe" are marked as synchronized When a synchronized method of an object is invoked, a thread takes out an object lock, or monitor If another thread attempts to execute any synchronized method, it finds that it is locked, and enters a state of suspension until the lock on the object monitor is released If several threads attempt to execute a method on a locked object, a queue of suspended threads will form When the thread that instituted the lock returns from the method, only one of the queued threads may access the object the release of a monitor does not allow more than one object to take out a new monitor One should note, however, that if a method is not synchronized and is executed while the object is locked, the thread will not block and the method can be run Method-level synchronization is a common mechanism for synchronizing access to resources When designing multi-threaded applications, or classes that need to be thread-safe for use in multi-threaded environments, methods can be synchronized to prevent data loss or corruption Unless data can be accessed atomically (in one operation, without the possibility of a thread being suspended by the JVM and control given to another), synchronization is needed Consider the simple example of a counter (such as of how many times an action like a hit on a Web page has occurred) stored in a file This counter may be incremented (by reading the current value and writing a new one) or read by multiple threads If one thread tries to increment the value of the
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counter before another thread completes modifying the counter, its value may be set by one and overwritten by the other This means that the counter would read an invalid value Worse still, if two attempts to overwrite the value were made, the file might be corrupted If the methods to access and modify the value of the counter were synchronized, however, only one thread could perform a write operation at any given moment The synchronized keyword is used to indicate that a method should be protected by a monitor Every method that could possibly be affected by concurrent access should be marked as synchronized This keyword should be used sparingly, however, as it has a performance drawback While fine for multi-threaded applications, in a single-threaded context it results in a waste of CPU time
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public class SomeClass { public synchronized void changeData( ) { } public synchronized Object getData ( ) { } }
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Let's look at a practical example of thread conflicts, and how they may be solved using synchronized methods Suppose we have a counter that can both be incremented and display a value If the method that provides access to the counter isn't thread-safe, and takes some time to complete, then two or more threads could access it at the same time and overwrite each increment Before the value of the counter can be stored, a second thread could write a new one, which is in turn overwritten (see Figure 7-3) This gets even more confusing when a read is made Since one update has been lost, an inaccurate tally is obtained If frequent changes are made to the counter, it becomes more and more inaccurate Figure 7-3 Concurrent access and modification of data by threads leads to data corruption
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The solution is to make the counter thread-safe, by synchronizing each method that performs a read or write operation If a synchronized method is used, only one thread can update the value at any given moment (see Figure 7-4) The thread that first invokes a synchronized method locks the object's monitor, which is released only when that method terminates No other thread can access any synchronized method of the counter object (though if multiple counters are used, this
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restriction applies only to individual counter instances, and not the Counter class itself) For this reason, synchronized methods should be as brief as possible threads that go to sleep inside a synchronized method will cause all other threads trying to invoke synchronized methods on that object to be suspended Figure 7-4 A thread-safe counter is achieved by synchronizing class methods
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Now we will look at method-level synchronization in action The following class defines a counter with synchronized access and modification methods
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// 7, Listing 6 public class Counter { private int countValue; public Counter() { countValue = 0; } public Counter(int start) { countValue = start; } // Synchronized method to increase counter public synchronized void increaseCount() { int count = countValue; // Simulate slow data processing and modification // Removing the synchronized keyword will demonstrate // how inaccurate concurrent access can be Adjusting // this value may be necessary on faster machines try { Threadsleep(5); } catch (InterruptedException ie) {} count = count + 1; countValue = count;
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} // Synchronized method to return counter value public synchronized int getCount() { return countValue; } }
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The next class is an application that uses multiple threads with a single counter Due to the synchronized keyword, concurrent modification of the counter's value is impossible but compiling and running the example without the presence of the synchronized keyword reveals some interesting results As predicted, the count becomes inaccurate However, as the write operation is so quick, it needs to be slowed down on faster machines to allow other threads time to enter the increaseCount() method Adjusting the sleep value may be required on very fast machines
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// 7, Listing 7 public class CountingThread implements Runnable { Counter myCounter; int countAmount; // Construct a counting thread to use the specified counter public CountingThread (Counter counter, int amount) { myCounter = counter; countAmount = amount; } public void run() { // Increase the counter the specified number of times for (int i = 1; i <= countAmount; i++) { // Increase the counter myCounterincreaseCount(); } } public static void main(String args[]) throws Exception { // Create a new, thread-safe counter Counter c = new Counter(); // Our runnable instance will increase the counter // ten times, for each thread that runs it Runnable runner = new CountingThread( c, 10 ); Systemoutprintln ("Starting counting threads"); Thread t1 = Thread t2 = Thread t3 = t1start(); new Thread(runner); new Thread(runner); new Thread(runner); t2start(); t3start();
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// Wait for all three threads to finish t1join(); t2join(); t3join(); Systemoutprintln ("Counter value is " + cgetCount() );
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Three threads are launched, which increment the counter 10 times each This means that, when the synchronized keyword is present, a total value of 30 will be returned by the counter's getCount() method Without the synchronized keyword, each thread overwrites the other and the resulting count is significantly less
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