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Truffle is a multi-language framework for executing dynamic languages that achieves high performance when combined with Graal.
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/*
 * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * The Universal Permissive License (UPL), Version 1.0
 *
 * Subject to the condition set forth below, permission is hereby granted to any
 * person obtaining a copy of this software, associated documentation and/or
 * data (collectively the "Software"), free of charge and under any and all
 * copyright rights in the Software, and any and all patent rights owned or
 * freely licensable by each licensor hereunder covering either (i) the
 * unmodified Software as contributed to or provided by such licensor, or (ii)
 * the Larger Works (as defined below), to deal in both
 *
 * (a) the Software, and
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 * (b) any piece of software and/or hardware listed in the lrgrwrks.txt file if
 * one is included with the Software each a "Larger Work" to which the Software
 * is contributed by such licensors),
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 * without restriction, including without limitation the rights to copy, create
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 * Software and the Larger Work(s), and to sublicense the foregoing rights on
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 * The above copyright notice and either this complete permission notice or at a
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 */
package com.oracle.truffle.api;

import static com.oracle.truffle.api.LanguageAccessor.ENGINE;

/**
 * A context local reference that refers to a value that is created for each polyglot context.
 * Context references can be used to attach data to a polyglot context. That data can be retrieved
 * efficiently for the current context using {@link #get()} or for other contexts using
 * {@link #get(TruffleContext)}. Context locals can be created for languages and instruments. See
 * links below for usage examples.
 *
 * @see TruffleLanguage#createContextLocal(com.oracle.truffle.api.TruffleLanguage.ContextLocalFactory)
 *      Usage with languages.
 * @see com.oracle.truffle.api.instrumentation.TruffleInstrument#createContextLocal(com.oracle.truffle.api.instrumentation.TruffleInstrument.ContextLocalFactory)
 *      Usage with instruments.
 * @since 20.3
 */
public abstract class ContextLocal {

    /**
     * Custom subclasses of context local are not allowed.
     *
     * @since 20.3
     */
    protected ContextLocal(Object polyglotObject) {
        if (!ENGINE.isPolyglotObject(polyglotObject)) {
            throw new IllegalStateException("No custom subclasses of ContextLocal allowed.");
        }
    }

    /**
     * Returns the context local value for the currently entered context. This method is intended to
     * be used on compiled code paths. The return value is never null.
     * 
     * @throws IllegalStateException if assertions (-ea) are enabled and no current context is
     *             entered on the current thread, context locals have already been cleared for the
     *             current context, the corresponding instrument is not initialized, or the
     *             corresponding language is not created.
     *
     * @since 20.3
     */
    public abstract T get();

    /**
     * Returns the context local value for an explicit {@link TruffleContext context}. This method
     * is intended to be used on compiled code paths. The return value is never null.
     * 
     * @throws IllegalStateException if assertions (-ea) are enabled and context locals have already
     *             been cleared for the specified context, the corresponding instrument is not
     *             initialized, or the corresponding language is not created.
     * 
     * @since 20.3
     */
    public abstract T get(TruffleContext context);
}