Java tutorial
/* * Copyright (C) 2007 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.base; import static com.google.common.base.Preconditions.checkNotNull; import static java.util.Arrays.asList; import static java.util.Collections.unmodifiableList; import com.google.common.annotations.Beta; import com.google.common.annotations.VisibleForTesting; import java.io.PrintWriter; import java.io.StringWriter; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.util.AbstractList; import java.util.ArrayList; import java.util.Collections; import java.util.List; import javax.annotation.CheckReturnValue; import javax.annotation.Nullable; /** * Static utility methods pertaining to instances of {@link Throwable}. * * <p>See the Guava User Guide entry on <a href= * "https://github.com/google/guava/wiki/ThrowablesExplained">Throwables</a>. * * @author Kevin Bourrillion * @author Ben Yu * @since 1.0 */ public final class Throwables { private Throwables() { } /** * Propagates {@code throwable} exactly as-is, if and only if it is an instance of {@code * declaredType}. Example usage: * <pre> * try { * someMethodThatCouldThrowAnything(); * } catch (IKnowWhatToDoWithThisException e) { * handle(e); * } catch (Throwable t) { * Throwables.propagateIfInstanceOf(t, IOException.class); * Throwables.propagateIfInstanceOf(t, SQLException.class); * throw Throwables.propagate(t); * } * </pre> */ public static <X extends Throwable> void propagateIfInstanceOf(@Nullable Throwable throwable, Class<X> declaredType) throws X { // Check for null is needed to avoid frequent JNI calls to isInstance(). if (throwable != null && declaredType.isInstance(throwable)) { throw declaredType.cast(throwable); } } /** * Propagates {@code throwable} exactly as-is, if and only if it is an instance of {@link * RuntimeException} or {@link Error}. Example usage: * <pre> * try { * someMethodThatCouldThrowAnything(); * } catch (IKnowWhatToDoWithThisException e) { * handle(e); * } catch (Throwable t) { * Throwables.propagateIfPossible(t); * throw new RuntimeException("unexpected", t); * } * </pre> */ public static void propagateIfPossible(@Nullable Throwable throwable) { propagateIfInstanceOf(throwable, Error.class); propagateIfInstanceOf(throwable, RuntimeException.class); } /** * Propagates {@code throwable} exactly as-is, if and only if it is an instance of {@link * RuntimeException}, {@link Error}, or {@code declaredType}. Example usage: * <pre> * try { * someMethodThatCouldThrowAnything(); * } catch (IKnowWhatToDoWithThisException e) { * handle(e); * } catch (Throwable t) { * Throwables.propagateIfPossible(t, OtherException.class); * throw new RuntimeException("unexpected", t); * } * </pre> * * @param throwable the Throwable to possibly propagate * @param declaredType the single checked exception type declared by the calling method */ public static <X extends Throwable> void propagateIfPossible(@Nullable Throwable throwable, Class<X> declaredType) throws X { propagateIfInstanceOf(throwable, declaredType); propagateIfPossible(throwable); } /** * Propagates {@code throwable} exactly as-is, if and only if it is an instance of {@link * RuntimeException}, {@link Error}, {@code declaredType1}, or {@code declaredType2}. In the * unlikely case that you have three or more declared checked exception types, you can handle them * all by invoking these methods repeatedly. See usage example in {@link * #propagateIfPossible(Throwable, Class)}. * * @param throwable the Throwable to possibly propagate * @param declaredType1 any checked exception type declared by the calling method * @param declaredType2 any other checked exception type declared by the calling method */ public static <X1 extends Throwable, X2 extends Throwable> void propagateIfPossible( @Nullable Throwable throwable, Class<X1> declaredType1, Class<X2> declaredType2) throws X1, X2 { checkNotNull(declaredType2); propagateIfInstanceOf(throwable, declaredType1); propagateIfPossible(throwable, declaredType2); } /** * Propagates {@code throwable} as-is if it is an instance of {@link RuntimeException} or {@link * Error}, or else as a last resort, wraps it in a {@code RuntimeException} and then propagates. * <p> * This method always throws an exception. The {@code RuntimeException} return type is only for * client code to make Java type system happy in case a return value is required by the enclosing * method. Example usage: * <pre> * T doSomething() { * try { * return someMethodThatCouldThrowAnything(); * } catch (IKnowWhatToDoWithThisException e) { * return handle(e); * } catch (Throwable t) { * throw Throwables.propagate(t); * } * } * </pre> * * @param throwable the Throwable to propagate * @return nothing will ever be returned; this return type is only for your convenience, as * illustrated in the example above */ public static RuntimeException propagate(Throwable throwable) { propagateIfPossible(checkNotNull(throwable)); throw new RuntimeException(throwable); } /** * Returns the innermost cause of {@code throwable}. The first throwable in a * chain provides context from when the error or exception was initially * detected. Example usage: * <pre> * assertEquals("Unable to assign a customer id", Throwables.getRootCause(e).getMessage()); * </pre> */ @CheckReturnValue public static Throwable getRootCause(Throwable throwable) { Throwable cause; while ((cause = throwable.getCause()) != null) { throwable = cause; } return throwable; } /** * Gets a {@code Throwable} cause chain as a list. The first entry in the list will be {@code * throwable} followed by its cause hierarchy. Note that this is a snapshot of the cause chain * and will not reflect any subsequent changes to the cause chain. * * <p>Here's an example of how it can be used to find specific types of exceptions in the cause * chain: * * <pre> * Iterables.filter(Throwables.getCausalChain(e), IOException.class)); * </pre> * * @param throwable the non-null {@code Throwable} to extract causes from * @return an unmodifiable list containing the cause chain starting with {@code throwable} */ @Beta // TODO(kevinb): decide best return type @CheckReturnValue public static List<Throwable> getCausalChain(Throwable throwable) { checkNotNull(throwable); List<Throwable> causes = new ArrayList<Throwable>(4); while (throwable != null) { causes.add(throwable); throwable = throwable.getCause(); } return Collections.unmodifiableList(causes); } /** * Returns a string containing the result of {@link Throwable#toString() toString()}, followed by * the full, recursive stack trace of {@code throwable}. Note that you probably should not be * parsing the resulting string; if you need programmatic access to the stack frames, you can call * {@link Throwable#getStackTrace()}. */ @CheckReturnValue public static String getStackTraceAsString(Throwable throwable) { StringWriter stringWriter = new StringWriter(); throwable.printStackTrace(new PrintWriter(stringWriter)); return stringWriter.toString(); } /** * Returns the stack trace of {@code throwable}, possibly providing slower iteration over the full * trace but faster iteration over parts of the trace. Here, "slower" and "faster" are defined in * comparison to the normal way to access the stack trace, {@link Throwable#getStackTrace() * throwable.getStackTrace()}. Note, however, that this method's special implementation is not * available for all platforms and configurations. If that implementation is unavailable, this * method falls back to {@code getStackTrace}. Callers that require the special implementation can * check its availability with {@link #lazyStackTraceIsLazy()}. * * <p>The expected (but not guaranteed) performance of the special implementation differs from * {@code getStackTrace} in one main way: The {@code lazyStackTrace} call itself returns quickly * by delaying the per-stack-frame work until each element is accessed. Roughly speaking: * * <ul> * <li>{@code getStackTrace} takes {@code stackSize} time to return but then negligible time to * retrieve each element of the returned list. * <li>{@code lazyStackTrace} takes negligible time to return but then {@code 1/stackSize} time to * retrieve each element of the returned list (probably slightly more than {@code 1/stackSize}). * </ul> * * <p>Note: The special implementation does not respect calls to {@link Throwable#setStackTrace * throwable.setStackTrace}. Instead, it always reflects the original stack trace from the * exception's creation. * * @since 19.0 */ // TODO(cpovirk): Say something about the possibility that List access could fail at runtime? @Beta @CheckReturnValue public static List<StackTraceElement> lazyStackTrace(Throwable throwable) { return lazyStackTraceIsLazy() ? jlaStackTrace(throwable) : unmodifiableList(asList(throwable.getStackTrace())); } /** * Returns whether {@link #lazyStackTrace} will use the special implementation described in its * documentation. * * @since 19.0 */ @Beta @CheckReturnValue public static boolean lazyStackTraceIsLazy() { return getStackTraceElementMethod != null & getStackTraceDepthMethod != null; } private static List<StackTraceElement> jlaStackTrace(final Throwable t) { checkNotNull(t); /* * TODO(cpovirk): Consider optimizing iterator() to catch IOOBE instead of doing bounds checks. * * TODO(cpovirk): Consider the UnsignedBytes pattern if it performs faster and doesn't cause * AOSP grief. */ return new AbstractList<StackTraceElement>() { @Override public StackTraceElement get(int n) { return (StackTraceElement) invokeAccessibleNonThrowingMethod(getStackTraceElementMethod, jla, t, n); } @Override public int size() { return (Integer) invokeAccessibleNonThrowingMethod(getStackTraceDepthMethod, jla, t); } }; } private static Object invokeAccessibleNonThrowingMethod(Method method, Object receiver, Object... params) { try { return method.invoke(receiver, params); } catch (IllegalAccessException e) { throw new RuntimeException(e); } catch (InvocationTargetException e) { throw propagate(e.getCause()); } } /** JavaLangAccess class name to load using reflection */ private static final String JAVA_LANG_ACCESS_CLASSNAME = "sun.misc.JavaLangAccess"; /** SharedSecrets class name to load using reflection */ @VisibleForTesting static final String SHARED_SECRETS_CLASSNAME = "sun.misc.SharedSecrets"; /** Access to some fancy internal JVM internals. */ @Nullable private static final Object jla = getJLA(); /** * The "getStackTraceElementMethod" method, only available on some JDKs so we use reflection to * find it when available. When this is null, use the slow way. */ @Nullable private static final Method getStackTraceElementMethod = (jla == null) ? null : getGetMethod(); /** * The "getStackTraceDepth" method, only available on some JDKs so we use reflection to find it * when available. When this is null, use the slow way. */ @Nullable private static final Method getStackTraceDepthMethod = (jla == null) ? null : getSizeMethod(); /** * Returns the JavaLangAccess class that is present in all Sun JDKs. It is not whitelisted for * AppEngine, and not present in non-Sun JDKs. */ @Nullable private static Object getJLA() { try { /* * We load sun.misc.* classes using reflection since Android doesn't support these classes and * would result in compilation failure if we directly refer to these classes. */ Class<?> sharedSecrets = Class.forName(SHARED_SECRETS_CLASSNAME, false, null); Method langAccess = sharedSecrets.getMethod("getJavaLangAccess"); return langAccess.invoke(null); } catch (ThreadDeath death) { throw death; } catch (Throwable t) { /* * This is not one of AppEngine's whitelisted classes, so even in Sun JDKs, this can fail with * a NoClassDefFoundError. Other apps might deny access to sun.misc packages. */ return null; } } /** * Returns the Method that can be used to resolve an individual StackTraceElement, or null if that * method cannot be found (it is only to be found in fairly recent JDKs). */ @Nullable private static Method getGetMethod() { return getJlaMethod("getStackTraceElement", Throwable.class, int.class); } /** * Returns the Method that can be used to return the size of a stack, or null if that method * cannot be found (it is only to be found in fairly recent JDKs). */ @Nullable private static Method getSizeMethod() { return getJlaMethod("getStackTraceDepth", Throwable.class); } @Nullable private static Method getJlaMethod(String name, Class<?>... parameterTypes) throws ThreadDeath { try { return Class.forName(JAVA_LANG_ACCESS_CLASSNAME, false, null).getMethod(name, parameterTypes); } catch (ThreadDeath death) { throw death; } catch (Throwable t) { /* * Either the JavaLangAccess class itself is not found, or the method is not supported on the * JVM. */ return null; } } }