dkaz / Generator.java
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| package net . abhinavsarkar . util ; |
| import java . util . Iterator ; |
| import java . util . concurrent . BrokenBarrierException ; |
| import java . util . concurrent . CyclicBarrier ; |
| import java . util . concurrent . ExecutorService ; |
| import java . util . concurrent . Executors ; |
| import java . util . concurrent . SynchronousQueue ; |
| import java . util . concurrent . ThreadFactory ; |
| import java . util . concurrent . TimeUnit ; |
| import java . util . concurrent . TimeoutException ; |
| import java . util . concurrent . atomic . AtomicBoolean ; |
| import com . google . common . base . Preconditions ; |
| import com . google . common . collect . AbstractIterator ; |
| /** |
| * Provides a Python-style Generator in Java. Subclass this class and |
| * implement the run method in subclass. Use to |
| * return a value and to receive a value. |
| * This class uses Preconditions and AbstractIterator classes from the Google |
| * Collections library. |
| * |
| * @author Abhinav Sarkar |
| */ |
| public abstract class Generator < T >implements Iterable < T > |
| private final int generatorNormalExitTimeout ; |
| private final int generatorAbnormalExitTimeout ; |
| private final SynchronousQueue < T >queue = new SynchronousQueue < T >(); |
| private final ExecutorService executorService = |
| Executors . newSingleThreadExecutor ( new DaemonThreadFactory ()); |
| private volatile Throwable throwable = null ; |
| private final AtomicBoolean started = new AtomicBoolean ( false ); |
| private final CyclicBarrier barrier = new CyclicBarrier ( 2 ); |
| private final Iterator < T >iterator = new AbstractIterator < T >() |
| @ Override |
| protected T computeNext () |
| T next = Generator . this . get (); |
| return next != null ? next : endOfData (); |
| >>; |
| /** |
| * Creates a Generator with defaults. |
| */ |
| public Generator () |
| this ( 1 , 2 ); |
| > |
| /** |
| * Creates a Generator. |
| * |
| * @param generatorNormalExitTimeout Timeout in seconds for the generator |
| * thread when the generator exits normally, either by finishing the run |
| * or by throwing a exception. |
| * @param generatorAbnormalExitTimeout Timeout in seconds for the generator |
| * thread when the generator exits abnormally by throwing any exception |
| * other than exception. |
| */ |
| public Generator ( int generatorNormalExitTimeout , |
| int generatorAbnormalExitTimeout ) |
| this . generatorNormalExitTimeout = generatorNormalExitTimeout ; |
| this . generatorAbnormalExitTimeout = generatorAbnormalExitTimeout ; |
| > |
| /* (non-Javadoc) |
| * @see java.lang.Iterable#iterator() |
| */ |
| public final Iterator < T >iterator () |
| return iterator ; |
| > |
| /** |
| * Gets a value from the generator. The generator must call |
| * to return this value inside the |
| * method. |
| * |
| * @return The value yielded by the generator. |
| */ |
| public final T get () |
| try |
| return next (); |
| > catch ( Throwable e ) |
| executorService . shutdownNow (); |
| if (!( e instanceof GeneratorExit )) |
| throw new RuntimeException ( e ); |
| > else |
| return null ; |
| > |
| > |
| > |
| /** |
| * Sends a value to the generator. The generator must call |
| * to receive this value inside the |
| * method. |
| * |
| * @param sent The value sent to the generator. |
| */ |
| public void send ( T sent ) |
| try |
| next ( Preconditions . checkNotNull ( sent )); |
| > catch ( Throwable e ) |
| executorService . shutdownNow (); |
| if ( e instanceof GeneratorExit ) |
| return ; |
| > else |
| throw new RuntimeException ( e ); |
| > |
| > |
| > |
| /** |
| * Stops the generator and frees up the background thread. |
| */ |
| public final void stop () |
| executorService . shutdownNow (); |
| > |
| /** |
| * Call this method inside method to yield a value. |
| * |
| * @param result Value to be yielded. |
| * @throws InterruptedException |
| * @throws BrokenBarrierException |
| */ |
| protected final void yield ( T result ) |
| throws InterruptedException , BrokenBarrierException |
| barrier . await (); |
| queue . put ( result ); |
| > |
| /** |
| * Call this method inside method to receive a value. |
| * |
| * @return The value sent to generator using |
| * method. |
| * @throws InterruptedException |
| * @throws BrokenBarrierException |
| */ |
| protected final T yield () |
| throws InterruptedException , BrokenBarrierException |
| barrier . await (); |
| return queue . take (); |
| > |
| private T next () throws Throwable |
| initialize (); |
| return queue . take (); |
| > |
| private void next ( T sent ) throws Throwable |
| initialize (); |
| queue . put ( sent ); |
| > |
| private void initialize () throws Throwable |
| if ( executorService . isShutdown ()) |
| if ( throwable != null ) |
| throw new IllegalStateException ( |
| «Generator has exited» , throwable ); |
| > else |
| throw new IllegalStateException ( «Generator has exited» ); |
| > |
| > |
| if (! started . get ()) |
| executorService . execute ( new Runnable () |
| public void run () |
| try |
| Generator . this . run (); |
| throw new GeneratorExit (); |
| > catch ( Exception e ) |
| Generator . this . throwable = e ; |
| try |
| if ( e instanceof GeneratorExit ) |
| barrier . await ( generatorNormalExitTimeout , |
| TimeUnit . SECONDS ); |
| > else |
| barrier . await ( generatorAbnormalExitTimeout , |
| TimeUnit . SECONDS ); |
| > |
| > catch ( InterruptedException e1 ) |
| return ; |
| > catch ( BrokenBarrierException e1 ) |
| return ; |
| > catch ( TimeoutException e1 ) |
| executorService . shutdownNow (); |
| > |
| > |
| >>); |
| started . set ( true ); |
| > |
| barrier . await (); |
| if ( throwable != null ) |
| throw throwable ; |
| > |
| > |
| /** |
| * Implement this method inside the subclass to provide the logic for the |
| * generator. |
| * |
| * @throws Exception |
| */ |
| protected abstract void run () throws Exception ; |
| private static final class DaemonThreadFactory |
| implements ThreadFactory |
| private final ThreadFactory defaultThreadFactory = |
| Executors . defaultThreadFactory (); |
| public Thread newThread ( Runnable r ) |
| Thread thread = defaultThreadFactory . newThread ( r ); |
| thread . setDaemon ( true ); |
| return thread ; |
| > |
| > |
| /** |
| * The Exception thrown to signal the exit of the generator. |
| * |
| * @author Abhinav Sarkar |
| */ |
| public static class GeneratorExit extends RuntimeException |
| private static final long serialVersionUID = 1L ; |
| > |
| > |
| /** |
| * A (infinite) Fibonacci number generator. |
| * |
| * @author Abhinav Sarkar |
| */ |
| class Fibonacci extends Generator < Integer > |
| /** |
| * Override the default constructor to set the timeout parameters. |
| */ |
| public Fibonacci () |
| super ( 1 , 5 ); |
| > |
| @ Override |
| protected void run () throws Exception |
| yield( 0 ); |
| int i = 0 ; |
| int j = 1 ; |
| while ( true ) |
| yield( j ); |
| int current = i + j ; |
| i = j ; |
| j = current ; |
| > |
| > |
| > |
| /** |
| * A (infinite) generator which prints the received values to |
| * stream. |
| * |
| * @author Abhinav Sarkar |
| */ |
| class Printer extends Generator < Object > |
| @ Override |
| protected void run () throws Exception |
| while ( true ) |
| System . out . println ( yield ()); |
| > |
| > |
| > |
| /** |
| * A (infinite) generator which returns back the sent value. |
| * |
| * @author Abhinav Sarkar |
| */ |
| class Identity extends Generator < Object > |
| @ Override |
| protected void run () throws Exception |
| while ( true ) |
| yield( yield ()); |
| > |
| > |
| /** |
| * A (non-infinite) generator which generates the numbers from 0 to 4. |
| * |
| * @author Abhinav Sarkar |
| */ |
| class FiveNumberGenerator extends Generator < Integer > |
| @ Override |
| protected void run () throws Exception |
| for ( int i = 0 ; i < 5 ; i ++) |
| yield( i ); |
| > |
| >; |
| > |
| class Main |
| public static void main ( String [] args ) |
| // Use get method to get the values yielded by the generator. |
| Fibonacci fibonacci = new Fibonacci (); |
| for ( int i = 0 ; i < 5 ; i ++) |
| System . out . println ( fibonacci . get ()); |
| > |
| fibonacci . stop (); |
| // Generator which generate values (and not only consume values) |
| // can be iterated directly. |
| Fibonacci fibonacciIter = new Fibonacci (); |
| int j = 0 ; |
| for ( Integer integer : fibonacciIter ) |
| System . out . println ( integer ); |
| if ( j ++ > 10 ) break ; |
| > |
| fibonacciIter . stop (); |
| // Use send method to send the value to the generator. |
| Printer printer = new Printer (); |
| for ( int i = 1 ; i <= 5 ; i ++) |
| printer . send ( i ); |
| > |
| printer . stop (); |
| // Use both send and get methods together. Their counterparts (yield() |
| // and yield(Object) must be called in the same order as these are |
| // called here. Otherwise it will result in a deadlock. |
| Identity identity = new Identity (); |
| for ( int i = 0 ; i < 5 ; i ++) |
| identity . send ( i ); |
| System . out . println ( identity . get ()); |
| > |
| identity . stop (); |
| // Non infinite generators do not need to be stopped at the end of |
| // iteration. They stop automatically. |
| FiveNumberGenerator fiveNumberGenerator = new FiveNumberGenerator (); |
| for ( Integer integer : fiveNumberGenerator ) |
| System . out . println ( integer ); |
| > |
| > |
| > |