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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 get() or for other contexts using get(TruffleContext). Context locals can be created for languages and instruments. See links below for usage examples. 
See Also:

     Usage with languages.

     Usage with instruments.
Since:
20.3/**
 * 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<T> {

    
Custom subclasses of context local are not allowed.
Since:
20.3/**
     * 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 no current context is entered on the current thread. 
Since:
20.3/**
     * 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 <code>null</code>. Throws
     * {@link IllegalStateException} if no current context is entered on the current thread.
     *
     * @since 20.3
     */
    public abstract T get();

    
Returns the context local value for an explicit context. This method is intended to be used on compiled code paths. The return value is never null. Throws IllegalStateException if no current context is entered on the current thread. 
Since:
20.3/**
     * 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 <code>null</code>.
     * Throws {@link IllegalStateException} if no current context is entered on the current thread.
     *
     * @since 20.3
     */
    public abstract T get(TruffleContext context);

}