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Concurrency
Processes and Threads
Thread Objects
Defining and Starting a Thread
Pausing Execution with Sleep
Interrupts
Joins
The SimpleThreads Example
Synchronization
Thread Interference
Memory Consistency Errors
Synchronized Methods
Intrinsic Locks and Synchronization
Atomic Access
Liveness
Deadlock
Starvation and Livelock
Guarded Blocks
Immutable Objects
A Synchronized Class Example
A Strategy for Defining Immutable Objects
High Level Concurrency Objects
Lock Objects
Executors
Executor Interfaces
Thread Pools
Fork/Join
Concurrent Collections
Atomic Variables
Concurrent Random Numbers
For Further Reading
Questions and Exercises
Trail: Essential Classes
Lesson: Concurrency
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# Guarded Blocks

Threads often have to coordinate their actions. The most common coordination idiom is the guarded block. Such a block begins by polling a condition that must be true before the block can proceed. There are a number of steps to follow in order to do this correctly.

Suppose, for example `guardedJoy` is a method that must not proceed until a shared variable `joy` has been set by another thread. Such a method could, in theory, simply loop until the condition is satisfied, but that loop is wasteful, since it executes continuously while waiting.

    public void guardedJoy() {
        // Simple loop guard. Wastes
        // processor time. Don't do this!
        while(!joy) {}
        System.out.println("Joy has been achieved!");
    }

A more efficient guard invokes `Object.wait` to suspend the current thread. The invocation of `wait` does not return until another thread has issued a notification that some special event may have occurred — though not necessarily the event this thread is waiting for:

    public synchronized void guardedJoy() {
        // This guard only loops once for each special event, which may not
        // be the event we're waiting for.
        while(!joy) {
            try {
                wait();
            } catch (InterruptedException e) {}
        }
        System.out.println("Joy and efficiency have been achieved!");
    }

Note: Always invoke `wait` inside a loop that tests for the condition being waited for. Don't assume that the interrupt was for the particular condition you were waiting for, or that the condition is still true.

Like many methods that suspend execution, `wait` can throw `InterruptedException`. In this example, we can just ignore that exception — we only care about the value of `joy`.

Why is this version of `guardedJoy` synchronized? Suppose `d` is the object we're using to invoke `wait`. When a thread invokes `d.wait`, it must own the intrinsic lock for `d` — otherwise an error is thrown. Invoking `wait` inside a synchronized method is a simple way to acquire the intrinsic lock.

When `wait` is invoked, the thread releases the lock and suspends execution. At some future time, another thread will acquire the same lock and invoke `Object.notifyAll`, informing all threads waiting on that lock that something important has happened:

    public synchronized notifyJoy() {
        joy = true;
        notifyAll();
    }

Some time after the second thread has released the lock, the first thread reacquires the lock and resumes by returning from the invocation of `wait`.

Note: There is a second notification method, `notify`, which wakes up a single thread. Because `notify` doesn't allow you to specify the thread that is woken up, it is useful only in massively parallel applications — that is, programs with a large number of threads, all doing similar chores. In such an application, you don't care which thread gets woken up.

Let's use guarded blocks to create a Producer-Consumer application. This kind of application shares data between two threads: the producer, that creates the data, and the consumer, that does something with it. The two threads communicate using a shared object. Coordination is essential: the consumer thread must not attempt to retrieve the data before the producer thread has delivered it, and the producer thread must not attempt to deliver new data if the consumer hasn't retrieved the old data.

In this example, the data is a series of text messages, which are shared through an object of type ``Drop``:

    public class Drop {
        // Message sent from producer
        // to consumer.
        private String message;
        // True if consumer should wait
        // for producer to send message,
        // false if producer should wait for
        // consumer to retrieve message.
        private boolean empty = true;

        public synchronized String take() {
            // Wait until message is
            // available.
            while (empty) {
                try {
                    wait();
                } catch (InterruptedException e) {}
            }
            // Toggle status.
            empty = true;
            // Notify producer that
            // status has changed.
            notifyAll();
            return message;
        }

        public synchronized void put(String message) {
            // Wait until message has
            // been retrieved.
            while (!empty) {
                try {
                    wait();
                } catch (InterruptedException e) {}
            }
            // Toggle status.
            empty = false;
            // Store message.
            this.message = message;
            // Notify consumer that status
            // has changed.
            notifyAll();
        }
    }

The producer thread, defined in ``Producer``, sends a series of familiar messages. The string "DONE" indicates that all messages have been sent. To simulate the unpredictable nature of real-world applications, the producer thread pauses for random intervals between messages.

    import java.util.Random;

    public class Producer implements Runnable {
        private Drop drop;

        public Producer(Drop drop) {
            this.drop = drop;
        }

        public void run() {
            String importantInfo[] = {
                "Mares eat oats",
                "Does eat oats",
                "Little lambs eat ivy",
                "A kid will eat ivy too"
            };
            Random random = new Random();

            for (int i = 0;
                 i < importantInfo.length;
                 i++) {
                drop.put(importantInfo[i]);
                try {
                    Thread.sleep(random.nextInt(5000));
                } catch (InterruptedException e) {}
            }
            drop.put("DONE");
        }
    }

The consumer thread, defined in ``Consumer``, simply retrieves the messages and prints them out, until it retrieves the "DONE" string. This thread also pauses for random intervals.

    import java.util.Random;

    public class Consumer implements Runnable {
        private Drop drop;

        public Consumer(Drop drop) {
            this.drop = drop;
        }

        public void run() {
            Random random = new Random();
            for (String message = drop.take();
                 ! message.equals("DONE");
                 message = drop.take()) {
                System.out.format("MESSAGE RECEIVED: %s%n", message);
                try {
                    Thread.sleep(random.nextInt(5000));
                } catch (InterruptedException e) {}
            }
        }
    }

Finally, here is the main thread, defined in ``ProducerConsumerExample``, that launches the producer and consumer threads.

    public class ProducerConsumerExample {
        public static void main(String[] args) {
            Drop drop = new Drop();
            (new Thread(new Producer(drop))).start();
            (new Thread(new Consumer(drop))).start();
        }
    }

Note: The `Drop` class was written in order to demonstrate guarded blocks. To avoid re-inventing the wheel, examine the existing data structures in the Java Collections Framework before trying to code your own data-sharing objects. For more information, refer to the Questions and Exercises section.

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