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8.2 Synchronizing Multiple Threads

The correct behavior of a multithreaded program generally depends on multiple threads cooperating with each other. This often involves threads not doing certain things at the same time or waiting for each other to perform certain tasks. This type of cooperation is called synchronization. This section discusses some common strategies for synchronization and how they can be implemented in Java.

The simplest strategy for ensuring that threads are correctly synchronized is to write code that works correctly when executed concurrently by any number of threads. However, this is more easily said than done. Most useful computations involve doing some activity, such as updating an instance variable or updating a display, that must be synchronized in order to happen correctly.

If a method only updates its local variables and calls other methods that only modify their local variables, the method can be invoked by multiple threads without any need for synchronization. Math.sqrt() and the length() method of the String class are examples of such methods.

A method that creates objects and meets the above criterion may not require synchronization. If the constructors invoked by the method do not modify anything but their own local variables and instance variables of the object they are constructing, and they only call methods that do not need to be synchronized, the method itself does not need to be synchronized. An example of such a method is the substring() in the String class.

Beyond these two simple cases, it is impossible to give an exhaustive list of rules that can tell you whether or not a method needs to be synchronized. You need to consider what the method is doing and think about any ill effects of concurrent execution in order to decide if synchronization is necessary.

Single-Threaded Execution

When more than one thread is trying to update the same data at the same time, the result may be wrong or inconsistent. Consider the following example:

class CountIt {
    int i = 0;
    void count() {
        i = i + 1;
    }
}

The method count() is supposed to increment the variable i by one. However, suppose that there are two threads, A and B, that call count() at the same time. In this case, it is possible that i could be incremented only once, instead of twice. Say the value of i is 7. Thread A calls the count() method and computes i+1 as 8. Then thread B calls the count() method and computes i+1 as 8 because thread A has not yet assigned the new value to i. Next, thread A assigns the value 8 to the variable i. Finally, thread B assigns the value 8 to the variable i. Thus, even though the count() method is called twice, the variable has only been incremented once when the sequence is finished.

Clearly, this code can fail to produce its intended result when it is executed concurrently by more than one thread. A piece of code that can fail to produce its intended result when executed concurrently is called a critical section. However, a critical section does behave correctly when it is executed by only one thread at a time. The strategy of single-threaded execution is to allow only one thread to execute a critical section of code at a time. If a thread wants to execute a critical section that another thread is already executing, the thread has to wait until the first thread is done and no other thread is executing that code before it can proceed.

Java provides the synchronized statement and the synchronized method modifier for implementing single-threaded execution. Before executing the block in a synchronized statement, the current thread must obtain a lock for the object referenced by the expression. If a method is declared with the synchronized modifer, the current thread must obtain a lock before it can invoke the method. If the method is not declared static, the thread must obtain a lock associated with the object used to access the method. If the method is declared static, the thread must obtain a lock associated with the class in which the method is declared. Because a thread must obtain a lock before executing a synchronized method, Java guarantees that synchronized methods are executed by only one thread at a time.

Modifying the count() method to make it a synchronized method ensures that it works as intended.

class CountIt {
    int i = 0;
    synchronized void count() {
        i = i + 1;
    }
}

The strategy of single-threaded execution can also be used when multiple methods update the same data. Consider the following example:

class CountIt2 {
    int i = 0;
    void count() {
        i = i + 1;
    }
    void count2() {
        i = i + 2;
    }
}

By the same logic used above, if the count() and count2() methods are executed concurrently, the result could be to increment i by 1, 2, or 3. Both the count() and count2() methods can be declared as synchronized to ensure that they are not executed concurrently with themselves or each other:

class CountIt2 {
    int i = 0;
    synchronized void count() {
        i = i + 1;
    }
    synchronized void count2() {
        i = i + 2;
    }
}

Sometimes it's necessary for a thread to make multiple method calls to manipulate an object without another thread calling that object's methods at the same time. Consider the following example:

System.out.print(new Date());
System.out.print(" : ");
System.out.println(foo());

If the code in the example is executed concurrently by multiple threads, the output from the two threads will be interleaved. The synchronized keyword provides a way to ensure that only one thread at a time can execute a block of code. Before executing the block in a synchronized statement, the current thread must obtain a lock for the object referenced by the expression. The above code can be modified to give a thread exclusive access to the OutputStream object referenced by System.out:

synchronized (System.out) {
    System.out.print(new Date());
    System.out.print(" : ");
    System.out.println(foo());
}

Note that this approach only works if other code that wants to call methods in the same object also uses similar synchronized statements, or if the methods in question are all synchronized methods. In this case, the print() and println() methods are synchronized, so other pieces of code that need to use these methods do not need to use a synchronized statement.

When an inner class is updating fields in its enclosing instance, simply making a method synchronized does not provide the needed single-threaded execution. Consider the following code:

public class Z extends Frame {
    int pressCount = 0;
    ...
    private class CountButton extends Button
                              implements ActionListener {
        public void actionPerformed(ActionEvent evt) {
            pressCount ++;
        }
    } 
    ...
} 

If a Z object instantiates more than one instance of CountButton, you need to use single-threaded execution to ensure that updates to pressCount are done correctly. Unfortunately, declaring the actionPerformed() method of CountButton to be synchronized does not accomplish that goal because it only forces the method to acquire a lock on the instance of CountButton it is associated with before it executes. The object you need to acquire a lock for is the enclosing instance of Z.

One way to have a CountButton object capture a lock on its enclosing instance of Z is to update pressCount inside of a synchronized statement. For example:

synchronized (Z.this) {
    pressCount ++;
}

The drawback to this approach is that every piece of code that accesses pressCount in any inner class of Z must be in a similar synchronized statement. Otherwise, it is possible for pressCount to be updated incorrectly. The more pieces of code that need to be inside of synchronized statements, the more places there are to introduce bugs in your program.

A more robust approach is to have the inner class update a field in its enclosing instance by calling a synchronized method in the enclosing instance. For example:

public class Z extends Frame {
    int pressCount = 0;
    synchronized incrementPressCount() {
        pressCount++;
    } 
    ...
    private class CountButton extends Button
                              implements ActionListener {
        public void actionPerformed(ActionEvent evt) {
            incrementPressCount();
        } 
    } 
    ...
} 

References Inner Classes; Method modifiers; The synchronized Statement

Optimistic Single-Threaded Execution

When multiple threads are updating a data structure, single-threaded execution is the obvious strategy to use to ensure correctness of the operations on the data structure. However, single-threaded execution can cause some problems of its own. Consider the following example:

public class Queue extends java.util.Vector {
    synchronized public void put(Object obj) {
        addElement(obj);
    }
    synchronized public Object get() throws EmptyQueueException {
        if (size() == 0)
            throw new EmptyQueueException();
        Object obj = elementAt(0);
        removeElementAt(0);
        return obj;
    }
}

This example implements a first-in, first-out (FIFO) queue. If the get() method of a Queue object is called when the queue is empty, the method throws an exception. Now suppose that you want to write the get() method so that when the queue is empty, the method waits for an item to be put in the queue, rather than throwing an exception. In order for an item to be put in the queue, the put() method of the queue must be invoked. But using the single-threaded execution strategy, the put() method will never be able to run while the get() method is waiting for the queue to receive an item. A good way to solve this dilemma is to use a strategy called optimistic single-threaded execution.

The optimistic single-threaded execution strategy is similar to the single-threaded execution strategy. They both begin by getting a lock on an object to ensure that the currently executing thread is the only thread that can execute a piece of code, and they both end by releasing that lock. The difference is what happens in between. Using the optimistic single-threaded execution strategy, if a piece of code discovers that conditions are not right to proceed, the code releases the lock it has on the object that enforces single-threaded execution and waits. When another piece of code changes things in such a way that might allow the first piece of code to proceed, it notifies the first piece of code that it should try to regain the lock and proceed.

To implement this strategy, the Object class provides methods called wait(), notify(), and notifyAll(). These methods are inherited by every other class in Java. The following example shows how to implement a queue that uses the optimistic single-threaded execution strategy, so that when the queue is empty, its get() method waits for the queue to have an item put in it:

public class Queue extends java.util.Vector {
    synchronized public void put(Object obj) {
        addElement(obj);
        notify();
    }
    synchronized public Object get() throws EmptyQueueException {
        while (size() == 0)
            wait();
        Object obj = elementAt(0);
        removeElementAt(0);
        return obj;
    }
}

In the above implementation of the Queue class, the get() method calls wait() when the queue is empty. The wait() method releases the lock that excludes other threads from executing methods in the Queue object, and then waits until another thread calls the put() method. When put() is called, it adds an item to the queue and calls notify(). The notify() method tells a thread that is waiting to return from a wait() method that it should attempt to regain its lock and proceed. If there is more than one thread waiting to regain the lock on the object, notify() chooses one of the threads arbitrarily. The notifyAll() method is similar to notify(), but instead of choosing one thread to notify, it notifies all of the threads that are waiting to regain the lock on the object.

Notice that the get() method calls wait() inside a while loop. Between the time that wait() is notified that it should try to regain its lock and the time it actually does regain the lock, another thread may have called the get() method and emptied the queue. The while loop guards against this situation.

References Method modifiers; Object; The synchronized Statement

Rendezvous

Sometimes it is necessary to have a thread wait to continue until another thread has completed its work and died. This type of synchronization uses the rendezvous strategy. The Thread class provides the join() method for implementing this strategy. When the join() method is called on a Thread object, the method returns immediately if the thread is dead. Otherwise, the method waits until the thread dies and then returns.

References Thread

Balking

Some methods should not be executed concurrently, and have a time-sensitive nature that makes postponing calls to them a bad idea. This is a common situation when software is controlling real-world devices. Suppose you have a Java program that is embedded in an electronic control for a toilet. There is a method called flush() that is responsible for flushing a toilet, and flush() can be called from more than one thread. If a thread calls flush() while another thread is already executing flush(), the second call should do nothing. A toilet is capable of only one flush at a time, and having a concurrent call to the flush() method result in a second flush would only waste water.

This scenario suggests the use of the balking strategy. The balking strategy allows no more than one thread to execute a method at a time. If another thread attempts to execute the method, the method simply returns without doing anything. Here is an example that shows what such a flush() method might look like:

boolean busy;
void flush() {
    synchronized (this) {
        if (busy)
            return;
        busy = true;
    }
    // code to make flush happen goes here
    busy = false;
}

Explicit Synchronization

When the synchronization needs of a thread are not known in advance, you can use a strategy called explicit synchronization. The explicit synchronization strategy allows you to explicitly tell a thread when it can and cannot run. For example, you may want an animation to start and stop in response to external events that happen at unpredictable times, so you need to be able to tell the animation when it can run.

To implement this strategy, the Thread class provides methods called suspend() and resume(). You can suspend the execution of a thread by calling the suspend() method of the Thread object that controls the thread. You can later resume execution of the thread by calling the resume() method on the Thread object.

References Thread


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