common/io/Closer.java

/*
 * Copyright (C) 2012 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.io;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Throwables;

import java.io.Closeable;
import java.io.IOException;
import java.lang.reflect.Method;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.logging.Level;

import javax.annotation.Nullable;

/**
 * A {@link Closeable} that collects {@code Closeable} resources and closes them all when it is
 * {@linkplain #close closed}. This is intended to approximately emulate the behavior of Java 7's
 * <a href="http://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html">
 * try-with-resources</a> statement in JDK6-compatible code. Running on Java 7, code using this
 * should be approximately equivalent in behavior to the same code written with try-with-resources.
 * Running on Java 6, exceptions that cannot be thrown must be logged rather than being added to the
 * thrown exception as a suppressed exception.
 *
 * <p>This class is intended to be used in the following pattern:
 *
 * <pre>   {@code
 *   Closer closer = Closer.create();
 *   try {
 *     InputStream in = closer.register(openInputStream());
 *     OutputStream out = closer.register(openOutputStream());
 *     // do stuff
 *   } catch (Throwable e) {
 *     // ensure that any checked exception types other than IOException that could be thrown are
 *     // provided here, e.g. throw closer.rethrow(e, CheckedException.class);
 *     throw closer.rethrow(e);
 *   } finally {
 *     closer.close();
 *   }}</pre>
 *
 * <p>Note that this try-catch-finally block is not equivalent to a try-catch-finally block using
 * try-with-resources. To get the equivalent of that, you must wrap the above code in <i>another</i>
 * try block in order to catch any exception that may be thrown (including from the call to
 * {@code close()}).
 *
 * <p>This pattern ensures the following:
 *
 * <ul>
 *   <li>Each {@code Closeable} resource that is successfully registered will be closed later.</li>
 *   <li>If a {@code Throwable} is thrown in the try block, no exceptions that occur when attempting
 *   to close resources will be thrown from the finally block. The throwable from the try block will
 *   be thrown.</li>
 *   <li>If no exceptions or errors were thrown in the try block, the <i>first</i> exception thrown
 *   by an attempt to close a resource will be thrown.</li>
 *   <li>Any exception caught when attempting to close a resource that is <i>not</i> thrown
 *   (because another exception is already being thrown) is <i>suppressed</i>.</li>
 * </ul>
 *
 * <p>An exception that is suppressed is not thrown. The method of suppression used depends on the
 * version of Java the code is running on:
 *
 * <ul>
 *   <li><b>Java 7+:</b> Exceptions are suppressed by adding them to the exception that <i>will</i>
 *   be thrown using {@code Throwable.addSuppressed(Throwable)}.</li>
 *   <li><b>Java 6:</b> Exceptions are suppressed by logging them instead.</li>
 * </ul>
 *
 * @author Colin Decker
 * @since 14.0
 */
// Coffee's for {@link Closer closers} only.
@Beta
public final class Closer implements Closeable {

  /**
   * The suppressor implementation to use for the current Java version.
   */
  private static final Suppressor SUPPRESSOR = SuppressingSuppressor.isAvailable()
      ? SuppressingSuppressor.INSTANCE
      : LoggingSuppressor.INSTANCE;

  /**
   * Creates a new {@link Closer}.
   */
  public static Closer create() {
    return new Closer(SUPPRESSOR);
  }

  @VisibleForTesting final Suppressor suppressor;

  // only need space for 2 elements in most cases, so try to use the smallest array possible
  private final Deque<Closeable> stack = new ArrayDeque<Closeable>(4);
  private Throwable thrown;

  @VisibleForTesting Closer(Suppressor suppressor) {
    this.suppressor = checkNotNull(suppressor); // checkNotNull to satisfy null tests
  }

  /**
   * Registers the given {@code closeable} to be closed when this {@code Closer} is
   * {@linkplain #close closed}.
   *
   * @return the given {@code closeable}
   */
  // close. this word no longer has any meaning to me.
  public <C extends Closeable> C register(@Nullable C closeable) {
    if (closeable != null) {
      stack.addFirst(closeable);
    }

    return closeable;
  }

  /**
   * Stores the given throwable and rethrows it. It will be rethrown as is if it is an
   * {@code IOException}, {@code RuntimeException} or {@code Error}. Otherwise, it will be rethrown
   * wrapped in a {@code RuntimeException}. <b>Note:</b> Be sure to declare all of the checked
   * exception types your try block can throw when calling an overload of this method so as to avoid
   * losing the original exception type.
   *
   * <p>This method always throws, and as such should be called as
   * {@code throw closer.rethrow(e);} to ensure the compiler knows that it will throw.
   *
   * @return this method does not return; it always throws
   * @throws IOException when the given throwable is an IOException
   */
  public RuntimeException rethrow(Throwable e) throws IOException {
    checkNotNull(e);
    thrown = e;
    Throwables.propagateIfPossible(e, IOException.class);
    throw new RuntimeException(e);
  }

  /**
   * Stores the given throwable and rethrows it. It will be rethrown as is if it is an
   * {@code IOException}, {@code RuntimeException}, {@code Error} or a checked exception of the
   * given type. Otherwise, it will be rethrown wrapped in a {@code RuntimeException}. <b>Note:</b>
   * Be sure to declare all of the checked exception types your try block can throw when calling an
   * overload of this method so as to avoid losing the original exception type.
   *
   * <p>This method always throws, and as such should be called as
   * {@code throw closer.rethrow(e, ...);} to ensure the compiler knows that it will throw.
   *
   * @return this method does not return; it always throws
   * @throws IOException when the given throwable is an IOException
   * @throws X when the given throwable is of the declared type X
   */
  public <X extends Exception> RuntimeException rethrow(Throwable e,
      Class<X> declaredType) throws IOException, X {
    checkNotNull(e);
    thrown = e;
    Throwables.propagateIfPossible(e, IOException.class);
    Throwables.propagateIfPossible(e, declaredType);
    throw new RuntimeException(e);
  }

  /**
   * Stores the given throwable and rethrows it. It will be rethrown as is if it is an
   * {@code IOException}, {@code RuntimeException}, {@code Error} or a checked exception of either
   * of the given types. Otherwise, it will be rethrown wrapped in a {@code RuntimeException}.
   * <b>Note:</b> Be sure to declare all of the checked exception types your try block can throw
   * when calling an overload of this method so as to avoid losing the original exception type.
   *
   * <p>This method always throws, and as such should be called as
   * {@code throw closer.rethrow(e, ...);} to ensure the compiler knows that it will throw.
   *
   * @return this method does not return; it always throws
   * @throws IOException when the given throwable is an IOException
   * @throws X1 when the given throwable is of the declared type X1
   * @throws X2 when the given throwable is of the declared type X2
   */
  public <X1 extends Exception, X2 extends Exception> RuntimeException rethrow(
      Throwable e, Class<X1> declaredType1, Class<X2> declaredType2) throws IOException, X1, X2 {
    checkNotNull(e);
    thrown = e;
    Throwables.propagateIfPossible(e, IOException.class);
    Throwables.propagateIfPossible(e, declaredType1, declaredType2);
    throw new RuntimeException(e);
  }

  /**
   * Closes all {@code Closeable} instances that have been added to this {@code Closer}. If an
   * exception was thrown in the try block and passed to one of the {@code exceptionThrown} methods,
   * any exceptions thrown when attempting to close a closeable will be suppressed. Otherwise, the
   * <i>first</i> exception to be thrown from an attempt to close a closeable will be thrown and any
   * additional exceptions that are thrown after that will be suppressed.
   */
  @Override
  public void close() throws IOException {
    Throwable throwable = thrown;

    // close closeables in LIFO order
    while (!stack.isEmpty()) {
      Closeable closeable = stack.removeFirst();
      try {
        closeable.close();
      } catch (Throwable e) {
        if (throwable == null) {
          throwable = e;
        } else {
          suppressor.suppress(closeable, throwable, e);
        }
      }
    }

    if (thrown == null && throwable != null) {
      Throwables.propagateIfPossible(throwable, IOException.class);
      throw new AssertionError(throwable); // not possible
    }
  }

  /**
   * Suppression strategy interface.
   */
  @VisibleForTesting interface Suppressor {
    /**
     * Suppresses the given exception ({@code suppressed}) which was thrown when attempting to close
     * the given closeable. {@code thrown} is the exception that is actually being thrown from the
     * method. Implementations of this method should not throw under any circumstances.
     */
    void suppress(Closeable closeable, Throwable thrown, Throwable suppressed);
  }

  /**
   * Suppresses exceptions by logging them.
   */
  @VisibleForTesting static final class LoggingSuppressor implements Suppressor {

    static final LoggingSuppressor INSTANCE = new LoggingSuppressor();

    @Override
    public void suppress(Closeable closeable, Throwable thrown, Throwable suppressed) {
      // log to the same place as Closeables
      Closeables.logger.log(Level.WARNING,
          "Suppressing exception thrown when closing " + closeable, suppressed);
    }
  }

  /**
   * Suppresses exceptions by adding them to the exception that will be thrown using JDK7's
   * addSuppressed(Throwable) mechanism.
   */
  @VisibleForTesting static final class SuppressingSuppressor implements Suppressor {

    static final SuppressingSuppressor INSTANCE = new SuppressingSuppressor();

    static boolean isAvailable() {
      return addSuppressed != null;
    }

    static final Method addSuppressed = getAddSuppressed();

    private static Method getAddSuppressed() {
      try {
        return Throwable.class.getMethod("addSuppressed", Throwable.class);
      } catch (Throwable e) {
        return null;
      }
    }

    @Override
    public void suppress(Closeable closeable, Throwable thrown, Throwable suppressed) {
      // ensure no exceptions from addSuppressed
      if (thrown == suppressed) {
        return;
      }
      try {
        addSuppressed.invoke(thrown, suppressed);
      } catch (Throwable e) {
        // if, somehow, IllegalAccessException or another exception is thrown, fall back to logging
        LoggingSuppressor.INSTANCE.suppress(closeable, thrown, suppressed);
      }
    }
  }
}