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Classes for building Guice scopes, that get automatically transferred when dispatching work to other threads

There is a newer version: 10.0

// Copyright (c) Piotr Morgwai Kotarbinski, Licensed under the Apache License, Version 2.0
package pl.morgwai.base.guice.scopes;

import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

import com.google.inject.Key;
import com.google.inject.Provider;



/**
 * Stores attributes associated with some server-side processing/call (such as a servlet request
 * processing, an RPC or a session combining several received calls) and allows to execute
 * operations within itself.
 * 
 * Overriding classes must use themselves as {@code CtxT} type argument.
 * 
 * Overriding classes usually add properties and methods specific to a given type of call, like
 * given call's arguments etc.
 * 
 * If multiple threads run within the same context, the attributes that they access must be
 * thread-safe or properly synchronized.
 */
public abstract class ServerSideContext> {



	final ConcurrentMap, Object> attributes = new ConcurrentHashMap<>();
	final ContextTracker tracker;



	protected ServerSideContext(ContextTracker tracker) {
		this.tracker = tracker;
	}



	/**
	 * Sets itself as the current context for the current thread and executes {@code operation}
	 * synchronously. Afterwards clears the current context.
	 *
	 * @see ContextTrackingExecutor#executeWithinAll(java.util.List, Runnable)
	 */
	@SuppressWarnings("unchecked")
	public void executeWithinSelf(Runnable operation) {
		tracker.trackWhileExecuting((CtxT) this, operation);
	}



	/**
	 * Sets itself as the current context for the current thread and executes {@code operation}
	 * synchronously. Afterwards clears the current context.
	 *
	 * @see ContextTrackingExecutor#executeWithinAll(java.util.List, Callable)
	 */
	@SuppressWarnings("unchecked")
	public  T executeWithinSelf(Callable operation) throws Exception {
		return tracker.trackWhileExecuting((CtxT) this, operation);
	}



	/**
	 * Removes the attribute given by key from this context. This is sometimes useful
	 * to force the associated {@link ContextScope} to obtain a new instance from the unscoped
	 * provider if the current one is not usable anymore (for example a timed-out connection, etc).
	 * 
	 * Note: If multiple threads run within the same context, care must be taken to prevent
	 * some of them from retaining the old stale instance.
	 */
	public void removeAttribute(Key key) {
		attributes.remove(key);
	}



	/**
	 * Obtains the attribute given by {@code key}. If it is not yet present in this context,
	 * asks {@code provider} for a new instance and stores the result for subsequent calls.
	 * For internal use by {@link ContextScope#scope(Key, com.google.inject.Provider)}.
	 */
	@SuppressWarnings("unchecked")
	 T provideAttributeIfAbsent(Key key, Provider provider) {
		return (T) attributes.computeIfAbsent(key, (ignored) -> provider.get());
	}
}