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