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/*
* Copyright (C) 2010 The Android Open Source Project
*
* 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 dalvik.system;
/**
* CloseGuard is a mechanism for flagging implicit finalizer cleanup of
* resources that should have been cleaned up by explicit close
* methods (aka "explicit termination methods" in Effective Java).
* <p>
* A simple example: <pre> {@code
* class Foo {
*
* private final CloseGuard guard = CloseGuard.get();
*
* ...
*
* public Foo() {
* ...;
* guard.open("cleanup");
* }
*
* public void cleanup() {
* guard.close();
* ...;
* }
*
* protected void finalize() throws Throwable {
* try {
* if (guard != null) {
* guard.warnIfOpen();
* }
* cleanup();
* } finally {
* super.finalize();
* }
* }
* }
* }</pre>
*
* In usage where the resource to be explicitly cleaned up are
* allocated after object construction, CloseGuard protection can
* be deferred. For example: <pre> {@code
* class Bar {
*
* private final CloseGuard guard = CloseGuard.get();
*
* ...
*
* public Bar() {
* ...;
* }
*
* public void connect() {
* ...;
* guard.open("cleanup");
* }
*
* public void cleanup() {
* guard.close();
* ...;
* }
*
* protected void finalize() throws Throwable {
* try {
* if (guard != null) {
* guard.warnIfOpen();
* }
* cleanup();
* } finally {
* super.finalize();
* }
* }
* }
* }</pre>
*
* When used in a constructor calls to {@code open} should occur at
* the end of the constructor since an exception that would cause
* abrupt termination of the constructor will mean that the user will
* not have a reference to the object to cleanup explicitly. When used
* in a method, the call to {@code open} should occur just after
* resource acquisition.
*
* <p>
*
* Note that the null check on {@code guard} in the finalizer is to
* cover cases where a constructor throws an exception causing the
* {@code guard} to be uninitialized.
*
* @hide
*/
public final class CloseGuard {
/**
* Instance used when CloseGuard is disabled to avoid allocation.
*/
private static final CloseGuard NOOP = new CloseGuard();
/**
* Enabled by default so we can catch issues early in VM startup.
* Note, however, that Android disables this early in its startup,
* but enables it with DropBoxing for system apps on debug builds.
*/
private static volatile boolean ENABLED = true;
/**
* Hook for customizing how CloseGuard issues are reported.
*/
private static volatile Reporter REPORTER = new DefaultReporter();
/**
* Returns a CloseGuard instance. If CloseGuard is enabled, {@code
* #open(String)} can be used to set up the instance to warn on
* failure to close. If CloseGuard is disabled, a non-null no-op
* instance is returned.
*/
public static CloseGuard get() {
if (!ENABLED) {
return NOOP;
}
return new CloseGuard();
}
/**
* Used to enable or disable CloseGuard. Note that CloseGuard only
* warns if it is enabled for both allocation and finalization.
*/
public static void setEnabled(boolean enabled) {
ENABLED = enabled;
}
/**
* Used to replace default Reporter used to warn of CloseGuard
* violations. Must be non-null.
*/
public static void setReporter(Reporter reporter) {
if (reporter == null) {
throw new NullPointerException("reporter == null");
}
REPORTER = reporter;
}
/**
* Returns non-null CloseGuard.Reporter.
*/
public static Reporter getReporter() {
return REPORTER;
}
private CloseGuard() {}
/**
* If CloseGuard is enabled, {@code open} initializes the instance
* with a warning that the caller should have explicitly called the
* {@code closer} method instead of relying on finalization.
*
* @param closer non-null name of explicit termination method
* @throws NullPointerException if closer is null, regardless of
* whether or not CloseGuard is enabled
*/
public void open(String closer) {
// always perform the check for valid API usage...
if (closer == null) {
throw new NullPointerException("closer == null");
}
// ...but avoid allocating an allocationSite if disabled
if (this == NOOP || !ENABLED) {
return;
}
String message = "Explicit termination method '" + closer + "' not called";
allocationSite = new Throwable(message);
}
private Throwable allocationSite;
/**
* Marks this CloseGuard instance as closed to avoid warnings on
* finalization.
*/
public void close() {
allocationSite = null;
}
/**
* If CloseGuard is enabled, logs a warning if the caller did not
* properly cleanup by calling an explicit close method
* before finalization. If CloseGuard is disable, no action is
* performed.
*/
public void warnIfOpen() {
if (allocationSite == null || !ENABLED) {
return;
}
String message =
("A resource was acquired at attached stack trace but never released. "
+ "See java.io.Closeable for information on avoiding resource leaks.");
REPORTER.report(message, allocationSite);
}
/**
* Interface to allow customization of reporting behavior.
*/
public static interface Reporter {
public void report (String message, Throwable allocationSite);
}
/**
* Default Reporter which reports CloseGuard violations to the log.
*/
private static final class DefaultReporter implements Reporter {
public void report (String message, Throwable allocationSite) {
System.logW(message, allocationSite);
}
}
}
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