Coverage Report - org.apache.commons.lang3.concurrent.BackgroundInitializer
 
Classes in this File Line Coverage Branch Coverage Complexity
BackgroundInitializer
96%
32/33
100%
10/10
1,933
BackgroundInitializer$InitializationTask
100%
6/6
100%
2/2
1,933
 
 1  
 /*
 2  
  * Licensed to the Apache Software Foundation (ASF) under one or more
 3  
  * contributor license agreements.  See the NOTICE file distributed with
 4  
  * this work for additional information regarding copyright ownership.
 5  
  * The ASF licenses this file to You under the Apache License, Version 2.0
 6  
  * (the "License"); you may not use this file except in compliance with
 7  
  * the License.  You may obtain a copy of the License at
 8  
  *
 9  
  *      http://www.apache.org/licenses/LICENSE-2.0
 10  
  *
 11  
  * Unless required by applicable law or agreed to in writing, software
 12  
  * distributed under the License is distributed on an "AS IS" BASIS,
 13  
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 14  
  * See the License for the specific language governing permissions and
 15  
  * limitations under the License.
 16  
  */
 17  
 package org.apache.commons.lang3.concurrent;
 18  
 
 19  
 import java.util.concurrent.Callable;
 20  
 import java.util.concurrent.ExecutionException;
 21  
 import java.util.concurrent.ExecutorService;
 22  
 import java.util.concurrent.Executors;
 23  
 import java.util.concurrent.Future;
 24  
 
 25  
 /**
 26  
  * <p>
 27  
  * A class that allows complex initialization operations in a background task.
 28  
  * </p>
 29  
  * <p>
 30  
  * Applications often have to do some expensive initialization steps when they
 31  
  * are started, e.g. constructing a connection to a database, reading a
 32  
  * configuration file, etc. Doing these things in parallel can enhance
 33  
  * performance as the CPU load can be improved. However, when access to the
 34  
  * resources initialized in a background thread is actually required,
 35  
  * synchronization has to be performed to ensure that their initialization is
 36  
  * complete.
 37  
  * </p>
 38  
  * <p>
 39  
  * This abstract base class provides support for this use case. A concrete
 40  
  * subclass must implement the {@link #initialize()} method. Here an arbitrary
 41  
  * initialization can be implemented, and a result object can be returned. With
 42  
  * this method in place the basic usage of this class is as follows (where
 43  
  * {@code MyBackgroundInitializer} is a concrete subclass):
 44  
  * </p>
 45  
  *
 46  
  * <pre>
 47  
  * MyBackgroundInitializer initializer = new MyBackgroundInitializer();
 48  
  * initializer.start();
 49  
  * // Now do some other things. Initialization runs in a parallel thread
 50  
  * ...
 51  
  * // Wait for the end of initialization and access the result object
 52  
  * Object result = initializer.get();
 53  
  * </pre>
 54  
  *
 55  
  * <p>
 56  
  * After the construction of a {@code BackgroundInitializer} object its
 57  
  * {@link #start()} method has to be called. This starts the background
 58  
  * processing. The application can now continue to do other things. When it
 59  
  * needs access to the object produced by the {@code BackgroundInitializer} it
 60  
  * calls its {@link #get()} method. If initialization is already complete,
 61  
  * {@link #get()} returns the result object immediately. Otherwise it blocks
 62  
  * until the result object is fully constructed.
 63  
  * </p>
 64  
  * <p>
 65  
  * {@code BackgroundInitializer} is a thin wrapper around a {@code Future}
 66  
  * object and uses an {@code ExecutorService} for running the background
 67  
  * initialization task. It is possible to pass in an {@code ExecutorService} at
 68  
  * construction time or set one using {@code setExternalExecutor()} before
 69  
  * {@code start()} was called. Then this object is used to spawn the background
 70  
  * task. If no {@code ExecutorService} has been provided, {@code
 71  
  * BackgroundInitializer} creates a temporary {@code ExecutorService} and
 72  
  * destroys it when initialization is complete.
 73  
  * </p>
 74  
  * <p>
 75  
  * The methods provided by {@code BackgroundInitializer} provide for minimal
 76  
  * interaction with the wrapped {@code Future} object. It is also possible to
 77  
  * obtain the {@code Future} object directly. Then the enhanced functionality
 78  
  * offered by {@code Future} can be used, e.g. to check whether the background
 79  
  * operation is complete or to cancel the operation.
 80  
  * </p>
 81  
  *
 82  
  * @since 3.0
 83  
  * @param <T> the type of the object managed by this initializer class
 84  
  */
 85  
 public abstract class BackgroundInitializer<T> implements
 86  
         ConcurrentInitializer<T> {
 87  
     /** The external executor service for executing tasks. */
 88  
     private ExecutorService externalExecutor; // @GuardedBy("this")
 89  
 
 90  
     /** A reference to the executor service that is actually used. */
 91  
     private ExecutorService executor; // @GuardedBy("this")
 92  
 
 93  
     /** Stores the handle to the background task. */
 94  
     private Future<T> future;  // @GuardedBy("this")
 95  
 
 96  
     /**
 97  
      * Creates a new instance of {@code BackgroundInitializer}. No external
 98  
      * {@code ExecutorService} is used.
 99  
      */
 100  
     protected BackgroundInitializer() {
 101  80
         this(null);
 102  80
     }
 103  
 
 104  
     /**
 105  
      * Creates a new instance of {@code BackgroundInitializer} and initializes
 106  
      * it with the given {@code ExecutorService}. If the {@code ExecutorService}
 107  
      * is not null, the background task for initializing this object will be
 108  
      * scheduled at this service. Otherwise a new temporary {@code
 109  
      * ExecutorService} is created.
 110  
      *
 111  
      * @param exec an external {@code ExecutorService} to be used for task
 112  
      * execution
 113  
      */
 114  85
     protected BackgroundInitializer(final ExecutorService exec) {
 115  85
         setExternalExecutor(exec);
 116  85
     }
 117  
 
 118  
     /**
 119  
      * Returns the external {@code ExecutorService} to be used by this class.
 120  
      *
 121  
      * @return the {@code ExecutorService}
 122  
      */
 123  
     public final synchronized ExecutorService getExternalExecutor() {
 124  121
         return externalExecutor;
 125  
     }
 126  
 
 127  
     /**
 128  
      * Returns a flag whether this {@code BackgroundInitializer} has already
 129  
      * been started.
 130  
      *
 131  
      * @return a flag whether the {@link #start()} method has already been
 132  
      * called
 133  
      */
 134  
     public synchronized boolean isStarted() {
 135  266
         return future != null;
 136  
     }
 137  
 
 138  
     /**
 139  
      * Sets an {@code ExecutorService} to be used by this class. The {@code
 140  
      * ExecutorService} passed to this method is used for executing the
 141  
      * background task. Thus it is possible to re-use an already existing
 142  
      * {@code ExecutorService} or to use a specially configured one. If no
 143  
      * {@code ExecutorService} is set, this instance creates a temporary one and
 144  
      * destroys it after background initialization is complete. Note that this
 145  
      * method must be called before {@link #start()}; otherwise an exception is
 146  
      * thrown.
 147  
      *
 148  
      * @param externalExecutor the {@code ExecutorService} to be used
 149  
      * @throws IllegalStateException if this initializer has already been
 150  
      * started
 151  
      */
 152  
     public final synchronized void setExternalExecutor(
 153  
             final ExecutorService externalExecutor) {
 154  133
         if (isStarted()) {
 155  1
             throw new IllegalStateException(
 156  
                     "Cannot set ExecutorService after start()!");
 157  
         }
 158  
 
 159  132
         this.externalExecutor = externalExecutor;
 160  132
     }
 161  
 
 162  
     /**
 163  
      * Starts the background initialization. With this method the initializer
 164  
      * becomes active and invokes the {@link #initialize()} method in a
 165  
      * background task. A {@code BackgroundInitializer} can be started exactly
 166  
      * once. The return value of this method determines whether the start was
 167  
      * successful: only the first invocation of this method returns <b>true</b>,
 168  
      * following invocations will return <b>false</b>.
 169  
      *
 170  
      * @return a flag whether the initializer could be started successfully
 171  
      */
 172  
     public synchronized boolean start() {
 173  
         // Not yet started?
 174  83
         if (!isStarted()) {
 175  
 
 176  
             // Determine the executor to use and whether a temporary one has to
 177  
             // be created
 178  
             ExecutorService tempExec;
 179  73
             executor = getExternalExecutor();
 180  73
             if (executor == null) {
 181  23
                 executor = tempExec = createExecutor();
 182  
             } else {
 183  50
                 tempExec = null;
 184  
             }
 185  
 
 186  73
             future = executor.submit(createTask(tempExec));
 187  
 
 188  73
             return true;
 189  
         }
 190  
 
 191  10
         return false;
 192  
     }
 193  
 
 194  
     /**
 195  
      * Returns the result of the background initialization. This method blocks
 196  
      * until initialization is complete. If the background processing caused a
 197  
      * runtime exception, it is directly thrown by this method. Checked
 198  
      * exceptions, including {@code InterruptedException} are wrapped in a
 199  
      * {@link ConcurrentException}. Calling this method before {@link #start()}
 200  
      * was called causes an {@code IllegalStateException} exception to be
 201  
      * thrown.
 202  
      *
 203  
      * @return the object produced by this initializer
 204  
      * @throws ConcurrentException if a checked exception occurred during
 205  
      * background processing
 206  
      * @throws IllegalStateException if {@link #start()} has not been called
 207  
      */
 208  
     @Override
 209  
     public T get() throws ConcurrentException {
 210  
         try {
 211  115
             return getFuture().get();
 212  6
         } catch (final ExecutionException execex) {
 213  6
             ConcurrentUtils.handleCause(execex);
 214  0
             return null; // should not be reached
 215  1
         } catch (final InterruptedException iex) {
 216  
             // reset interrupted state
 217  1
             Thread.currentThread().interrupt();
 218  1
             throw new ConcurrentException(iex);
 219  
         }
 220  
     }
 221  
 
 222  
     /**
 223  
      * Returns the {@code Future} object that was created when {@link #start()}
 224  
      * was called. Therefore this method can only be called after {@code
 225  
      * start()}.
 226  
      *
 227  
      * @return the {@code Future} object wrapped by this initializer
 228  
      * @throws IllegalStateException if {@link #start()} has not been called
 229  
      */
 230  
     public synchronized Future<T> getFuture() {
 231  122
         if (future == null) {
 232  1
             throw new IllegalStateException("start() must be called first!");
 233  
         }
 234  
 
 235  121
         return future;
 236  
     }
 237  
 
 238  
     /**
 239  
      * Returns the {@code ExecutorService} that is actually used for executing
 240  
      * the background task. This method can be called after {@link #start()}
 241  
      * (before {@code start()} it returns <b>null</b>). If an external executor
 242  
      * was set, this is also the active executor. Otherwise this method returns
 243  
      * the temporary executor that was created by this object.
 244  
      *
 245  
      * @return the {@code ExecutorService} for executing the background task
 246  
      */
 247  
     protected synchronized final ExecutorService getActiveExecutor() {
 248  106
         return executor;
 249  
     }
 250  
 
 251  
     /**
 252  
      * Returns the number of background tasks to be created for this
 253  
      * initializer. This information is evaluated when a temporary {@code
 254  
      * ExecutorService} is created. This base implementation returns 1. Derived
 255  
      * classes that do more complex background processing can override it. This
 256  
      * method is called from a synchronized block by the {@link #start()}
 257  
      * method. Therefore overriding methods should be careful with obtaining
 258  
      * other locks and return as fast as possible.
 259  
      *
 260  
      * @return the number of background tasks required by this initializer
 261  
      */
 262  
     protected int getTaskCount() {
 263  49
         return 1;
 264  
     }
 265  
 
 266  
     /**
 267  
      * Performs the initialization. This method is called in a background task
 268  
      * when this {@code BackgroundInitializer} is started. It must be
 269  
      * implemented by a concrete subclass. An implementation is free to perform
 270  
      * arbitrary initialization. The object returned by this method can be
 271  
      * queried using the {@link #get()} method.
 272  
      *
 273  
      * @return a result object
 274  
      * @throws Exception if an error occurs
 275  
      */
 276  
     protected abstract T initialize() throws Exception;
 277  
 
 278  
     /**
 279  
      * Creates a task for the background initialization. The {@code Callable}
 280  
      * object returned by this method is passed to the {@code ExecutorService}.
 281  
      * This implementation returns a task that invokes the {@link #initialize()}
 282  
      * method. If a temporary {@code ExecutorService} is used, it is destroyed
 283  
      * at the end of the task.
 284  
      *
 285  
      * @param execDestroy the {@code ExecutorService} to be destroyed by the
 286  
      * task
 287  
      * @return a task for the background initialization
 288  
      */
 289  
     private Callable<T> createTask(final ExecutorService execDestroy) {
 290  73
         return new InitializationTask(execDestroy);
 291  
     }
 292  
 
 293  
     /**
 294  
      * Creates the {@code ExecutorService} to be used. This method is called if
 295  
      * no {@code ExecutorService} was provided at construction time.
 296  
      *
 297  
      * @return the {@code ExecutorService} to be used
 298  
      */
 299  
     private ExecutorService createExecutor() {
 300  23
         return Executors.newFixedThreadPool(getTaskCount());
 301  
     }
 302  
 
 303  
     private class InitializationTask implements Callable<T> {
 304  
         /** Stores the executor service to be destroyed at the end. */
 305  
         private final ExecutorService execFinally;
 306  
 
 307  
         /**
 308  
          * Creates a new instance of {@code InitializationTask} and initializes
 309  
          * it with the {@code ExecutorService} to be destroyed at the end.
 310  
          *
 311  
          * @param exec the {@code ExecutorService}
 312  
          */
 313  73
         public InitializationTask(final ExecutorService exec) {
 314  73
             execFinally = exec;
 315  73
         }
 316  
 
 317  
         /**
 318  
          * Initiates initialization and returns the result.
 319  
          *
 320  
          * @return the result object
 321  
          * @throws Exception if an error occurs
 322  
          */
 323  
         @Override
 324  
         public T call() throws Exception {
 325  
             try {
 326  73
                 return initialize();
 327  
             } finally {
 328  73
                 if (execFinally != null) {
 329  23
                     execFinally.shutdown();
 330  
                 }
 331  
             }
 332  
         }
 333  
     }
 334  
 }