95 lines
2.7 KiB
Java
95 lines
2.7 KiB
Java
// Copyright (C) 2011 The Android Open Source Project
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package com.google.gerrit.server.rules;
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import com.googlecode.prolog_cafe.exceptions.SystemException;
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import com.googlecode.prolog_cafe.lang.Prolog;
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/**
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* Defines a value cached in a {@link PrologEnvironment}.
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*
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* @see StoredValues
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*/
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public class StoredValue<T> {
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/** Construct a new unique key that does not match any other key. */
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public static <T> StoredValue<T> create() {
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return new StoredValue<>();
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}
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/** Construct a key based on a Java Class object, useful for singletons. */
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public static <T> StoredValue<T> create(Class<T> clazz) {
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return new StoredValue<>(clazz);
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}
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private final Object key;
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/**
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* Initialize a stored value key using any Java Object.
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*
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* @param key unique identity of the stored value. This will be the hash key in the Prolog
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* Environments's hash map.
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*/
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public StoredValue(Object key) {
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this.key = key;
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}
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/** Initializes a stored value key with a new unique key. */
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public StoredValue() {
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key = this;
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}
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/** Look up the value in the engine, or return null. */
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public T getOrNull(Prolog engine) {
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return get((PrologEnvironment) engine.control);
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}
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/** Get the value from the engine, or throw SystemException. */
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public T get(Prolog engine) {
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T obj = getOrNull(engine);
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if (obj == null) {
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//unless createValue() is overridden, will return null
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obj = createValue(engine);
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if (obj == null) {
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throw new SystemException("No " + key + " available");
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}
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set(engine, obj);
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}
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return obj;
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}
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public void set(Prolog engine, T obj) {
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set((PrologEnvironment) engine.control, obj);
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}
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/** Perform {@link #getOrNull(Prolog)} on the environment's interpreter. */
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public T get(PrologEnvironment env) {
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return env.get(this);
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}
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/** Set the value into the environment's interpreter. */
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public void set(PrologEnvironment env, T obj) {
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env.set(this, obj);
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}
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/**
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* Creates a value to store, returns null by default.
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*
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* @param engine Prolog engine.
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* @return new value.
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*/
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protected T createValue(Prolog engine) {
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return null;
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}
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}
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