| // |
| // Licensed to the Apache Software Foundation (ASF) under one |
| // or more contributor license agreements. See the NOTICE file |
| // distributed with this work for additional information |
| // regarding copyright ownership. The ASF licenses this file |
| // to you 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. |
| // |
| |
| = DevFaqLookupDefault |
| :jbake-type: wiki |
| :jbake-tags: wiki, devfaq, needsreview |
| :jbake-status: published |
| |
| === What is the "default lookup"? |
| |
| The default lookup is `Lookup.getDefault()`. |
| It is the registry for global singletons and instances of objects which have been registered in the system by modules. |
| (In JDK 6, `ServiceLoader` operates on the same principle.) |
| The default lookup searches in two places: |
| |
| * The link:http://bits.netbeans.org/dev/javadoc/org-openide-util-lookup/org/openide/util/lookup/Lookups.html#metaInfServices(java.lang.ClassLoader)[META-INF/services/ Lookup] contains all objects registered by modules via the link:http://java.sun.com/j2se/1.4/docs/guide/extensions/[Java Extension Mechanism] - putting files in the `META-INF/services/` directory of their module JARs (typically done using the [link:http://bits.netbeans.org/dev/javadoc/org-openide-util-lookup/org/openide/util/lookup/ServiceProvider.html[http://bits.netbeans.org/dev/javadoc/org-openide-util-lookup/org/openide/util/lookup/ServiceProvider.html] `@ServiceProvider`] annotation) |
| * The contents of the `Services/` folder of the link:DevFaqSystemFilesystem.html[System (configuration) Filesystem] (this is harder and somewhat deprecated) |
| |
| Objects contained in the default lookup are instantiated lazily when first requested. Objects returned by the default lookup _may_ (or may not) be garbage collected if they become unreferenced. |
| |
| Here is the usual usage pattern: |
| |
| 1. A central "controller" module defines some interface, e.g. |
| |
| [source,java] |
| ---- |
| |
| package controller.pkg; |
| public interface MyService { |
| void doSomething(); |
| } |
| |
| ---- |
| |
| 2. Each module which wants to implement that service depends on the controller module which defines the interface, and creates and registers an implementation: |
| |
| [source,java] |
| ---- |
| |
| @ServiceProvider(service=MyService.class) |
| public class MyImpl implements MyService { |
| public void doSomething() {....} |
| } |
| |
| ---- |
| |
| It is also possible to declaratively mask other people's implementations and declaratively order implementations so some will take precedence. |
| |
| 3. The controller finds all implementations and uses them somehow: |
| |
| [source,java] |
| ---- |
| |
| for (MyService s : Lookup.getDefault().lookupAll(MyService.class)) { |
| s.doSomething(); |
| } |
| |
| ---- |
| |
| === More About Lookup |
| |
| * link:DevFaqLookup.html[DevFaqLookup] |
| * link:http://bits.netbeans.org/dev/javadoc/usecases.html#usecase-Utilities[Extension Points Tutorial] |
| * link:http://bits.netbeans.org/dev/javadoc/org-openide-util-lookup/org/openide/util/Lookup.html[Javadoc for Lookup] |
| |
| <hr/> |
| |
| Applies to: NetBeans 6.7 and later |
| |
| === Apache Migration Information |
| |
| The content in this page was kindly donated by Oracle Corp. to the |
| Apache Software Foundation. |
| |
| This page was exported from link:http://wiki.netbeans.org/DevFaqLookupDefault[http://wiki.netbeans.org/DevFaqLookupDefault] , |
| that was last modified by NetBeans user Jtulach |
| on 2010-07-24T20:14:14Z. |
| |
| |
| *NOTE:* This document was automatically converted to the AsciiDoc format on 2018-01-10, and needs to be reviewed. |