https://projects.eclipse.org/projects/ee4j.jersey/jersey-common: Jersey core common packages (Eclipse Foundation) 
/*
 * Copyright (c) 2012, 2019 Oracle and/or its affiliates. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v. 2.0, which is available at
 * http://www.eclipse.org/legal/epl-2.0.
 *
 * This Source Code may also be made available under the following Secondary
 * Licenses when the conditions for such availability set forth in the
 * Eclipse Public License v. 2.0 are satisfied: GNU General Public License,
 * version 2 with the GNU Classpath Exception, which is available at
 * https://www.gnu.org/software/classpath/license.html.
 *
 * SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
 */

package org.glassfish.jersey.internal.util.collection;


A collection of Value provider factory & utility methods. 
Author:Marek Potociar
/**
 * A collection of {@link Value Value provider} factory & utility methods.
 *
 * @author Marek Potociar
 */
public final class Values {

    private static final LazyValue EMPTY = new LazyValue() {
        @Override
        public Object get() {
            return null;
        }

        @Override
        public boolean isInitialized() {
            return true;
        }
    };

    private static final LazyUnsafeValue EMPTY_UNSAFE = new LazyUnsafeValue() {
        @Override
        public Object get() {
            return null;
        }

        @Override
        public boolean isInitialized() {
            return true;
        }
    };

    private Values() {
        // prevents instantiation.
    }

    
Get an empty value provider whose get() method always returns null. 
Type parameters:
  • <T> – value type.
Returns:empty value provider.
/**
     * Get an empty {@link Value value provider} whose {@link Value#get() get()}
     * method always returns {@code null}.
     *
     * @param <T> value type.
     * @return empty value provider.
     */
    public static <T> Value<T> empty() {
        //noinspection unchecked
        return (Value<T>) EMPTY;
    }

    
Get an empty value provider whose get() method always returns null. 
Type parameters:
  • <T> – value type.
Returns:empty unsafe value provider.
/**
     * Get an empty {@link UnsafeValue value provider} whose {@link UnsafeValue#get() get()}
     * method always returns {@code null}.
     *
     * @param <T> value type.
     * @return empty unsafe value provider.
     */
    public static <T, E extends Throwable> UnsafeValue<T, E> emptyUnsafe() {
        //noinspection unchecked
        return (UnsafeValue<T, E>) EMPTY_UNSAFE;
    }

    
Get a new constant value provider whose get() method always returns the instance supplied to the value parameter. 
 In case the supplied value constant is null, an empty value provider is returned. 
Params:
  • value – value instance to be provided.
Type parameters:
  • <T> –   value type.
Returns:constant value provider.
/**
     * Get a new constant {@link Value value provider} whose {@link Value#get() get()}
     * method always returns the instance supplied to the {@code value} parameter.
     * <p/>
     * In case the supplied value constant is {@code null}, an {@link #empty() empty} value
     * provider is returned.
     *
     * @param <T>   value type.
     * @param value value instance to be provided.
     * @return constant value provider.
     */
    public static <T> Value<T> of(final T value) {
        return (value == null) ? Values.<T>empty() : new InstanceValue<T>(value);
    }

    private static class InstanceValue<T> implements Value<T> {

        private final T value;

        public InstanceValue(final T value) {
            this.value = value;
        }

        @Override
        public T get() {
            return value;
        }

        @Override
        public boolean equals(final Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }

            return value.equals(((InstanceValue) o).value);
        }

        @Override
        public int hashCode() {
            return value != null ? value.hashCode() : 0;
        }

        @Override
        public String toString() {
            return "InstanceValue{value=" + value + '}';
        }
    }

    
Get a new constant value provider whose get() method always returns the instance supplied to the value parameter. 
 In case the supplied value constant is null, an empty value provider is returned. 
Params:
  • value – value instance to be provided.
Type parameters:
  • <T> –   value type.
Returns:constant value provider.
/**
     * Get a new constant {@link UnsafeValue value provider} whose {@link UnsafeValue#get() get()}
     * method always returns the instance supplied to the {@code value} parameter.
     * <p/>
     * In case the supplied value constant is {@code null}, an {@link #emptyUnsafe() empty} value
     * provider is returned.
     *
     * @param <T>   value type.
     * @param value value instance to be provided.
     * @return constant value provider.
     */
    public static <T, E extends Throwable> UnsafeValue<T, E> unsafe(final T value) {
        return (value == null) ? Values.<T, E>emptyUnsafe() : new InstanceUnsafeValue<T, E>(value);
    }

    private static class InstanceUnsafeValue<T, E extends Throwable> implements UnsafeValue<T, E> {

        private final T value;

        public InstanceUnsafeValue(final T value) {
            this.value = value;
        }

        @Override
        public T get() {
            return value;
        }

        @Override
        public boolean equals(final Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }

            return value.equals(((InstanceUnsafeValue) o).value);
        }

        @Override
        public int hashCode() {
            return value != null ? value.hashCode() : 0;
        }

        @Override
        public String toString() {
            return "InstanceUnsafeValue{value=" + value + '}';
        }
    }

    
Get a new "throwing" unsafe value provider whose get() method always throws the exception supplied to the throwable parameter. 
 In case the supplied throwable is null, an NullPointerException is thrown. 
Params:
  • throwable – throwable instance to be thrown.
Type parameters:
  • <T> –       value type.
  • <E> –       exception type.
Throws:
Returns:"throwing" unsafe value provider.
/**
     * Get a new "throwing" {@link UnsafeValue unsafe value provider} whose {@link UnsafeValue#get() get()}
     * method always throws the exception supplied to the {@code throwable} parameter.
     * <p/>
     * In case the supplied throwable is {@code null}, an {@link NullPointerException} is thrown.
     *
     * @param <T>       value type.
     * @param <E>       exception type.
     * @param throwable throwable instance to be thrown.
     * @return "throwing" unsafe value provider.
     * @throws NullPointerException in case the supplied throwable instance is {@code null}.
     */
    public static <T, E extends Throwable> UnsafeValue<T, E> throwing(final E throwable) {
        if (throwable == null) {
            throw new NullPointerException("Supplied throwable ");
        }

        return new ExceptionValue<T, E>(throwable);
    }

    private static class ExceptionValue<T, E extends Throwable> implements UnsafeValue<T, E> {

        private final E throwable;

        public ExceptionValue(final E throwable) {
            this.throwable = throwable;
        }

        @Override
        public T get() throws E {
            throw throwable;
        }

        @Override
        public boolean equals(final Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }

            return throwable.equals(((ExceptionValue) o).throwable);
        }

        @Override
        public int hashCode() {
            return throwable != null ? throwable.hashCode() : 0;
        }

        @Override
        public String toString() {
            return "ExceptionValue{throwable=" + throwable + '}';
        }
    }

    
Get a new lazily initialized value provider. 
 The value returned by its get() method is lazily retrieved during a first call to the method from the supplied delegate value provider and is then cached for a subsequent retrieval. 
 The implementation of the returned lazy value provider is thread-safe and is guaranteed to invoke the get() method on the supplied delegate value provider instance at most once. 


 If the supplied value provider is null, an empty value provider is returned. 


Params:
  • delegate – value provider delegate that will be used to lazily initialize the value provider.
Type parameters:
  • <T> –      value type.
Returns:lazily initialized value provider.
/**
     * Get a new lazily initialized {@link Value value provider}.
     * <p/>
     * The value returned by its {@link Value#get() get()} method is lazily retrieved during a first
     * call to the method from the supplied {@code delegate} value provider and is then cached for
     * a subsequent retrieval.
     * <p>
     * The implementation of the returned lazy value provider is thread-safe and is guaranteed to
     * invoke the {@code get()} method on the supplied {@code delegate} value provider instance at
     * most once.
     * </p>
     * <p>
     * If the supplied value provider is {@code null}, an {@link #empty() empty} value
     * provider is returned.
     * </p>
     *
     * @param <T>      value type.
     * @param delegate value provider delegate that will be used to lazily initialize the value provider.
     * @return lazily initialized value provider.
     */
    public static <T> LazyValue<T> lazy(final Value<T> delegate) {
        //noinspection unchecked
        return (delegate == null) ? (LazyValue<T>) EMPTY : new LazyValueImpl<T>(delegate);
    }

    
Get a new eagerly initialized value provider. 
 The value returned by its get() method is eagerly computed from the supplied delegate value provider and is then stored in a final field for a subsequent retrieval. 
 The implementation of the returned eager value provider is thread-safe and is guaranteed to invoke the get() method on the supplied delegate value provider instance once and only once. 


 If the supplied value provider is null, an empty value provider is returned. 


Params:
  • delegate – value provider delegate that will be used to eagerly initialize the value provider.
Type parameters:
  • <T> –      value type.
Returns:eagerly initialized, constant value provider.
/**
     * Get a new eagerly initialized {@link Value value provider}.
     * <p/>
     * The value returned by its {@link Value#get() get()} method is eagerly computed from the supplied
     * {@code delegate} value provider and is then stored in a final field for a subsequent retrieval.
     * <p>
     * The implementation of the returned eager value provider is thread-safe and is guaranteed to
     * invoke the {@code get()} method on the supplied {@code delegate} value provider instance once
     * and only once.
     * </p>
     * <p>
     * If the supplied value provider is {@code null}, an {@link #empty() empty} value
     * provider is returned.
     * </p>
     *
     * @param <T>      value type.
     * @param delegate value provider delegate that will be used to eagerly initialize the value provider.
     * @return eagerly initialized, constant value provider.
     */
    public static <T> Value<T> eager(final Value<T> delegate) {
        return (delegate == null) ? Values.<T>empty() : new EagerValue<T>(delegate);
    }

    private static class EagerValue<T> implements Value<T> {

        private final T result;

        private EagerValue(final Value<T> value) {
            this.result = value.get();
        }

        @Override
        public T get() {
            return result;
        }
    }

    private static class LazyValueImpl<T> implements LazyValue<T> {

        private final Object lock;
        private final Value<T> delegate;

        private volatile Value<T> value;

        public LazyValueImpl(final Value<T> delegate) {
            this.delegate = delegate;
            this.lock = new Object();
        }

        @Override
        public T get() {
            Value<T> result = value;
            if (result == null) {
                synchronized (lock) {
                    result = value;
                    if (result == null) {
                        value = result = Values.of(delegate.get());
                    }
                }
            }
            return result.get();
        }

        @Override
        public boolean isInitialized() {
            return value != null;
        }

        @Override
        public boolean equals(final Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }

            return delegate.equals(((LazyValueImpl) o).delegate);
        }

        @Override
        public int hashCode() {
            return delegate != null ? delegate.hashCode() : 0;
        }

        @Override
        public String toString() {
            return "LazyValue{delegate=" + delegate.toString() + '}';
        }
    }

    
Get a new lazily initialized unsafe value provider. 
 The value returned by its get() method is lazily retrieved during a first call to the method from the supplied delegate value provider and is then cached for a subsequent retrieval. 
 In case the call to underlying delegate.get() throws a throwable, the throwable is cached and thrown on all subsequent retrievals. 


 The implementation of the returned lazy value provider is thread-safe and is guaranteed to invoke the get() method on the supplied delegate value provider instance at most once. 


 If the supplied value provider is null, an empty value provider is returned. 


Params:
  • delegate – value provider delegate that will be used to lazily initialize the value provider.
Type parameters:
  • <T> –      value type.
Returns:lazily initialized value provider.
/**
     * Get a new lazily initialized {@link UnsafeValue unsafe value provider}.
     * <p/>
     * The value returned by its {@link UnsafeValue#get() get()} method is lazily retrieved during a first
     * call to the method from the supplied {@code delegate} value provider and is then cached for
     * a subsequent retrieval.
     * <p>
     * In case the call to underlying {@code delegate.get()} throws a throwable, the throwable is cached
     * and thrown on all subsequent retrievals.
     * </p>
     * <p>
     * The implementation of the returned lazy value provider is thread-safe and is guaranteed to
     * invoke the {@code get()} method on the supplied {@code delegate} value provider instance at
     * most once.
     * </p>
     * <p>
     * If the supplied value provider is {@code null}, an {@link #empty() empty} value
     * provider is returned.
     * </p>
     *
     * @param <T>      value type.
     * @param delegate value provider delegate that will be used to lazily initialize the value provider.
     * @return lazily initialized value provider.
     */
    public static <T, E extends Throwable> LazyUnsafeValue<T, E> lazy(final UnsafeValue<T, E> delegate) {
        //noinspection unchecked
        return (delegate == null) ? (LazyUnsafeValue<T, E>) EMPTY_UNSAFE : new LazyUnsafeValueImpl<T, E>(delegate);
    }

    private static class LazyUnsafeValueImpl<T, E extends Throwable> implements LazyUnsafeValue<T, E> {

        private final Object lock;
        private final UnsafeValue<T, E> delegate;

        private volatile UnsafeValue<T, E> value;

        public LazyUnsafeValueImpl(final UnsafeValue<T, E> delegate) {
            this.delegate = delegate;
            this.lock = new Object();
        }

        @Override
        public T get() throws E {
            UnsafeValue<T, E> result = value;
            if (result == null) {
                synchronized (lock) {
                    result = value;
                    //noinspection ConstantConditions
                    if (result == null) {
                        try {
                            result = Values.unsafe(delegate.get());
                        } catch (final Throwable e) {
                            //noinspection unchecked
                            result = Values.throwing((E) e);
                        }
                        value = result;
                    }
                }
            }
            return result.get();
        }

        @Override
        public boolean isInitialized() {
            return value != null;
        }

        @Override
        public boolean equals(final Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }

            return delegate.equals(((LazyUnsafeValueImpl) o).delegate);
        }

        @Override
        public int hashCode() {
            return delegate != null ? delegate.hashCode() : 0;
        }

        @Override
        public String toString() {
            return "LazyValue{delegate=" + delegate.toString() + '}';
        }
    }
}