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/*
 * Copyright (C) 2009 The JSR-330 Expert Group
 *
 * 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.facebook.presto.jdbc.internal.inject;

import java.lang.annotation.Target;
import java.lang.annotation.Retention;
import java.lang.annotation.Documented;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import static java.lang.annotation.ElementType.ANNOTATION_TYPE;

/**
 * Identifies scope annotations. A scope annotation applies to a class
 * containing an injectable constructor and governs how the injector reuses
 * instances of the type. By default, if no scope annotation is present, the
 * injector creates an instance (by injecting the type's constructor), uses
 * the instance for one injection, and then forgets it. If a scope annotation
 * is present, the injector may retain the instance for possible reuse in a
 * later injection. If multiple threads can access a scoped instance, its
 * implementation should be thread safe. The implementation of the scope
 * itself is left up to the injector.
 *
 * 

In the following example, the scope annotation {@code @Singleton} ensures * that we only have one Log instance: * *

 *   @Singleton
 *   class Log {
 *     void log(String message) { ... }
 *   }
* *

The injector generates an error if it encounters more than one scope * annotation on the same class or a scope annotation it doesn't support. * *

A scope annotation: *

    *
  • is annotated with {@code @Scope}, {@code @Retention(RUNTIME)}, * and typically {@code @Documented}.
  • *
  • should not have attributes.
  • *
  • is typically not {@code @Inherited}, so scoping is orthogonal to * implementation inheritance.
  • *
  • may have restricted usage if annotated with {@code @Target}. While * this specification covers applying scopes to classes only, some * injector configurations might use scope annotations * in other places (on factory method results for example).
  • *
* *

For example: * *

 *   @java.lang.annotation.Documented
 *   @java.lang.annotation.Retention(RUNTIME)
 *   @com.facebook.presto.jdbc.internal.inject.Scope
 *   public @interface RequestScoped {}
* *

Annotating scope annotations with {@code @Scope} helps the injector * detect the case where a programmer used the scope annotation on a class but * forgot to configure the scope in the injector. A conservative injector * would generate an error rather than not apply a scope. * * @see com.facebook.presto.jdbc.internal.inject.Singleton @Singleton */ @Target(ANNOTATION_TYPE) @Retention(RUNTIME) @Documented public @interface Scope {}





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