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package org.graalvm.nativeimage;

import org.graalvm.nativeimage.hosted.Feature;
import org.graalvm.nativeimage.impl.ImageSingletonsSupport;

A key-value store of singleton objects. The registry is populated during native image generation (no changes are allowed at run time); it is queried both during native image generation and at run time. The key is a class or interface, the value is an instance that extends that class. 
 When accessing the registry at run time, the key must be a compile time constant. The lookup is performed during native image generation, and the resulting value is put into the compiled code as a literal constant. Therefore, the actual map that stores the key-value data is not necessary at run time, and there is no lookup overhead at run time. 
 ImageSingletons avoids static fields: instead of filling a static field during native image generation and accessing it at run time, the value is put into the ImageSingletons. Usually, that happens in one of the early initialization methods of a Feature. 
Since:
19.0/**
 * A key-value store of singleton objects. The registry is populated during native image generation
 * (no changes are allowed at run time); it is queried both during native image generation and at
 * run time. The key is a {@link java.lang.Class class or interface}, the value is an instance that
 * extends that class.
 * <p>
 * When accessing the registry at run time, the key must be a compile time constant. The
 * {@link #lookup} is performed during native image generation, and the resulting value is put into
 * the compiled code as a literal constant. Therefore, the actual map that stores the key-value data
 * is not necessary at run time, and there is no lookup overhead at run time.
 * <p>
 * {@link ImageSingletons} avoids static fields: instead of filling a static field during native
 * image generation and accessing it at run time, the value is put into the {@link ImageSingletons}.
 * Usually, that happens in one of the early initialization methods of a {@link Feature}.
 *
 * @since 19.0
 */
public final class ImageSingletons {

    
Add a singleton to the registry. The key must be unique, i.e., no value must have been
registered with the given class before.
Since:
19.0/**
     * Add a singleton to the registry. The key must be unique, i.e., no value must have been
     * registered with the given class before.
     *
     * @since 19.0
     */
    @Platforms(Platform.HOSTED_ONLY.class)
    public static <T> void add(Class<T> key, T value) {
        ImageSingletonsSupport.get().add(key, value);
    }

    
Lookup a singleton in the registry. The key must be a compile time constant, so that the call
to this method can be replaced with the constant configuration objects.
 The key must have been added before, i.e., the result is guaranteed to be non- null. 
Since:
19.0/**
     * Lookup a singleton in the registry. The key must be a compile time constant, so that the call
     * to this method can be replaced with the constant configuration objects.
     * <p>
     * The key must have been {@link #add added} before, i.e., the result is guaranteed to be non-
     * {@code null}.
     *
     * @since 19.0
     */
    public static <T> T lookup(Class<T> key) {
        return ImageSingletonsSupport.get().lookup(key);
    }

    
Checks if a singleton is in the registry. The key must be a compile time constant, so that the call to this method can be replaced with the constant true or false. 
 The method returns false since 19.3.0 when not used in the context of a native image. As such it is safe to write: {@codesnippet org.graalvm.nativeimage.ImageSingletonsTest} and let such code run fine in the context of native image as well as outside of it. 
Since:
19.0/**
     * Checks if a singleton is in the registry. The key must be a compile time constant, so that
     * the call to this method can be replaced with the constant {@code true} or {@code false}.
     * <p>
     * The method returns {@code false} since 19.3.0 when not used in the context of a native image.
     * As such it is safe to write:
     *
     * {@codesnippet org.graalvm.nativeimage.ImageSingletonsTest}
     *
     * and let such code run fine in the context of {@link ImageInfo#inImageCode() native image} as
     * well as outside of it.
     *
     * @since 19.0
     */
    public static boolean contains(Class<?> key) {
        if (!ImageInfo.inImageCode()) {
            return false;
        }
        return ImageSingletonsSupport.get().contains(key);
    }

    private ImageSingletons() {
    }
}