Java tutorial
/* * 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); } } } }