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package com.google.inject.internal;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.Iterables;
import com.google.common.collect.ListMultimap;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.inject.Injector;
import com.google.inject.Key;
import com.google.inject.Provider;
import com.google.inject.ProvisionException;
import com.google.inject.Scope;
import com.google.inject.Scopes;
import com.google.inject.Singleton;
import com.google.inject.internal.CycleDetectingLock.CycleDetectingLockFactory;
import com.google.inject.spi.Dependency;
import com.google.inject.spi.DependencyAndSource;
import com.google.inject.spi.Message;
import java.lang.ref.WeakReference;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentMap;
/**
* One instance per {@link Injector}. Also see {@code @}{@link Singleton}.
*
* Introduction from the author:
* Implementation of this class seems unreasonably complicated at the first sight.
* I fully agree with you, that the beast below is very complex
* and it's hard to reason on how does it work or not.
* Still I want to assure you that hundreds(?) of hours were thrown
* into making this code simple, while still maintaining Singleton contract.
*
* Anyway, why is it so complex? Singleton scope does not seem to be that unique.
* 1) Guice has never truly expected to be used in multi threading environment
* with many Injectors working alongside each other. There is almost no
* code with Guice that propagates state between threads. And Singleton
* scope is The exception.
* 2) Guice supports circular dependencies and thus manages proxy objects.
* There is no interface that allows user defined Scopes to create proxies,
* it is expected to be done by Guice. Singleton scope needs to be
* able to detect circular dependencies spanning several threads,
* therefore Singleton scope needs to be able to create these proxies.
* 3) To make things worse, Guice has a very tricky definition for a binding
* resolution when Injectors are in in a parent/child relationship.
* And Scope does not have access to this information by design,
* the only real action that Scope can do is to call or not to call a creator.
* 4) There is no readily available code in Guice that can detect a potential
* deadlock, and no code for handling dependency cycles spanning several threads.
* This is significantly harder as all the dependencies in a thread at runtime
* can be represented with a list, where in a multi threaded environment
* we have more complex dependency trees.
* 5) Guice has a pretty strong contract regarding Garbage Collection,
* which often prevents us from linking objects directly.
* So simple domain specific code can not be written and intermediary
* id objects need to be managed.
* 6) Guice is relatively fast and we should not make things worse.
* We're trying our best to optimize synchronization for speed and memory.
* Happy path should be almost as fast as in a single threaded solution
* and should not take much more memory.
* 7) Error message generation in Guice was not meant to be used like this and to work around
* its APIs we need a lot of code. Additional complexity comes from inherent data races
* as message is only generated when failure occurs on proxy object generation.
* Things get ugly pretty fast.
*
* @see #scope(Key, Provider)
* @see CycleDetectingLock
*
* @author timofeyb (Timothy Basanov)
*/
public class SingletonScope implements Scope {
/** A sentinel value representing null. */
private static final Object NULL = new Object();
/**
* SingletonScope needs the owning injector's thread-specific InternalContext object during
* singleton instantiation, to look up type information on the singleton instance, and to
* determine whether circular proxy creation is enabled within InjectorOptions.
*
* For additional complications: the owning injector's InternalContext can change between
* provider creation and actually instantiating the singleton, via calls to
* {@link InjectorImpl#callInContext}.
*
* A thread-specific reference to the owning injector is stored here, so that the singleton
* provider can access the correct InternalContext for the given thread during provider.get().
*
* The ThreadLocal stores WeakReference, so that the references here and inside the singleton
* provider do not interfere with garbage collection and post-collection cleanup of child
* injectors, happening in {@link WeakKeySet}.
*/
static final ThreadLocal> currentInjector =
new ThreadLocal>();
/**
* A map of thread running singleton instantiation, to the InternalContext that is relevant
* to the singleton being instantiated. In the case of a multithreaded circular dependency in
* singleton instantiation, this map is used to provide an extensive error message that contains
* information on all of the threads involved in the circular dependency.
*/
private static final ConcurrentMap internalContextsMap =
Maps.newConcurrentMap();
/**
* Allows us to detect when circular proxies are necessary. It's only used during singleton
* instance initialization, after initialization direct access through volatile field is used.
*
* NB: Factory uses {@link Key}s as a user locks ids, different injectors can
* share them. Cycles are detected properly as cycle detection does not rely on user locks ids,
* but error message generated could be less than ideal.
*
* TODO(user): we may use one factory per injector tree for optimization reasons
*/
private static final CycleDetectingLockFactory> cycleDetectingLockFactory =
new CycleDetectingLockFactory>();
/**
* Provides singleton scope with the following properties:
* - creates no more than one instance per Key as a creator is used no more than once,
* - result is cached and returned quickly on subsequent calls,
* - exception in a creator is not treated as instance creation and is not cached,
* - creates singletons in parallel whenever possible,
* - waits for dependent singletons to be created even across threads and when dependencies
* are shared as long as no circular dependencies are detected,
* - returns circular proxy only when circular dependencies are detected,
* - aside from that, blocking synchronization is only used for proxy creation and initialization,
* @see CycleDetectingLockFactory
*/
@Override
public Provider scope(final Key key, final Provider creator) {
/**
* Locking strategy:
* - volatile instance: double-checked locking for quick exit when scope is initialized,
* - constructionContext: manipulations with proxies list or instance initialization
* - creationLock: singleton instance creation,
* -- allows to guarantee only one instance per singleton,
* -- special type of a lock, that prevents potential deadlocks,
* -- guards constructionContext for all operations except proxy creation
*/
return new Provider() {
/**
* The lazily initialized singleton instance. Once set, this will either have type T or will
* be equal to NULL. Would never be reset to null.
*/
volatile Object instance;
/**
* Circular proxies are used when potential deadlocks are detected. Guarded by itself.
* ConstructionContext is not thread-safe, so each call should be synchronized.
*/
final ConstructionContext constructionContext = new ConstructionContext();
/** For each binding there is a separate lock that we hold during object creation. */
final CycleDetectingLock> creationLock = cycleDetectingLockFactory.create(key);
/**
* The singleton provider needs a reference back to the injector, in order to get ahold
* of InternalContext during instantiation.
*/
final InjectorImpl injector = currentInjector.get().get();
@SuppressWarnings("DoubleCheckedLocking")
@Override
public T get() {
// cache volatile variable for the usual case of already initialized object
final Object initialInstance = instance;
if (initialInstance == null) {
// instance is not initialized yet
// first, store the current InternalContext in a map, so that if there is a circular
// dependency error, we can use the InternalContext objects to create a complete
// error message.
final Thread currentThread = Thread.currentThread();
final InternalContext context = injector.getLocalContext();
final InternalContext previousContext = internalContextsMap.get(currentThread);
internalContextsMap.put(currentThread, context);
try {
// acquire lock for current binding to initialize an instance
final ListMultimap> locksCycle =
creationLock.lockOrDetectPotentialLocksCycle();
if (locksCycle.isEmpty()) {
// this thread now owns creation of an instance
try {
// intentionally reread volatile variable to prevent double initialization
if (instance == null) {
// creator throwing an exception can cause circular proxies created in
// different thread to never be resolved, just a warning
T provided = creator.get();
Object providedNotNull = provided == null ? NULL : provided;
// scope called recursively can initialize instance as a side effect
if (instance == null) {
// instance is still not initialized, so we can proceed
// don't remember proxies created by Guice on circular dependency
// detection within the same thread; they are not real instances to cache
if (Scopes.isCircularProxy(provided)) {
return provided;
}
synchronized (constructionContext) {
// guarantee thread-safety for instance and proxies initialization
instance = providedNotNull;
constructionContext.setProxyDelegates(provided);
}
} else {
// safety assert in case instance was initialized
Preconditions.checkState(instance == providedNotNull,
"Singleton is called recursively returning different results");
}
}
} catch (RuntimeException e) {
// something went wrong, be sure to clean a construction context
// this helps to prevent potential memory leaks in circular proxies list
synchronized (constructionContext) {
constructionContext.finishConstruction();
}
throw e;
} finally {
// always release our creation lock, even on failures
creationLock.unlock();
}
} else {
// potential deadlock detected, creation lock is not taken by this thread
synchronized (constructionContext) {
// guarantee thread-safety for instance and proxies initialization
if (instance == null) {
// creating a proxy to satisfy circular dependency across several threads
Dependency dependency = Preconditions.checkNotNull(
context.getDependency(), "internalContext.getDependency()");
Class rawType = dependency.getKey().getTypeLiteral().getRawType();
try {
@SuppressWarnings("unchecked")
T proxy = (T) constructionContext.createProxy(
new Errors(), context.getInjectorOptions(), rawType);
return proxy;
} catch (ErrorsException e) {
// best effort to create a rich error message
Message proxyCreationError =
Iterables.getOnlyElement(e.getErrors().getMessages());
Message cycleDependenciesMessage = createCycleDependenciesMessage(
ImmutableMap.copyOf(internalContextsMap),
locksCycle,
proxyCreationError);
// adding stack trace generated by us in addition to a standard one
throw new ProvisionException(ImmutableList.of(
cycleDependenciesMessage, proxyCreationError));
}
}
}
}
} finally {
// restore internalContextsMap to previous state, in order to support nested singleton
// construction spanning multiple injectors.
if (previousContext != null) {
internalContextsMap.put(currentThread, previousContext);
} else {
internalContextsMap.remove(currentThread);
}
}
// at this point we're sure that singleton was initialized,
// reread volatile variable to catch all corner cases
// caching volatile variable to minimize number of reads performed
final Object initializedInstance = instance;
Preconditions.checkState(initializedInstance != null,
"Internal error: Singleton is not initialized contrary to our expectations");
@SuppressWarnings("unchecked")
T initializedTypedInstance = (T) initializedInstance;
return initializedInstance == NULL ? null : initializedTypedInstance;
} else {
// singleton is already initialized and local cache can be used
@SuppressWarnings("unchecked")
T typedInitialIntance = (T) initialInstance;
return initialInstance == NULL ? null : typedInitialIntance;
}
}
/**
* Helper method to create beautiful and rich error descriptions. Best effort and slow.
* Tries its best to provide dependency information from injectors currently available
* in a global internal context.
*
* The main thing being done is creating a list of Dependencies involved into
* lock cycle across all the threads involved. This is a structure we're creating:
*
* { Current Thread, C.class, B.class, Other Thread, B.class, C.class, Current Thread }
* To be inserted in the beginning by Guice: { A.class, B.class, C.class }
*
* When we're calling Guice to create A and it fails in the deadlock while trying to
* create C, which is being created by another thread, which waits for B. List would
* be reversed before printing it to the end user.
*/
private Message createCycleDependenciesMessage(
Map internalContextsMap,
ListMultimap> locksCycle,
Message proxyCreationError) {
// this is the main thing that we'll show in an error message,
// current thread is populate by Guice
List