<answer id="arch-overall">
<p>
This module defines the <a href="@TOP@/org/openide/util/Lookup.html">
Lookup</a> which is the NetBeans way for dynamic registration
and lookup of
components in our modularized component system. It allows lookup and discovery
of features by description of their interfaces. The classes are devided
into two parts. The
<api group="java" name="LookupAPI"type="export" category="official"url="@TOP@/org/openide/util/lookup/doc-files/lookup-api.html">
allows the discovery</api> and the
<api group="java" name="LookupSPI"type="export" category="official"url="@TOP@/org/openide/util/lookup/doc-files/lookup-spi.html">
simplifies creation and registration of own lookup objects</api>.
</p>
</answer>
There is a great introduction to Lookup and its usage in its
<a href="@TOP@/org/openide/util/Lookup.html">javadoc</a>.
For details on this topic,
together with code samples, see chapter 4,
of <a href="https://leanpub.com/nbp4beginners">NetBeans Platform for Beginners</a>
by Jason Wexbridge and Walter Nyland.
In addition to that here is
a list of frequently asked or interesting questions slowly expanding as
people ask them:
<h3>Lookup faq:</h3>
<usecase id="lookup-on-certain-platform" name="How to specify that a service in Lookup should be available only on Windows?" >
<em><b>Q:</b>
Most of the time I specify interfaces that I want to add to the Lookup class in the layer.xml file.
But, let's say I have a platform-specific interface (something on Windows only, for instance).</em>
<p>
<em>
How can I specify (in the xml, or programmatically) that this service should only be added to the Lookup if the platform is Windows?
</em>>
</p>
In general there are three ways to achieve this.
<ul>
<li><p>It is possible to write a specific module and enable it only on windows.
See <a href="@org-openide-modules@/org/openide/modules/doc-files/api.html#how-os-specific">os specific modules</a> documentation.
Then you can put a registration of your instance into your module's
<a href="@org-openide-util@/org/openide/util/doc-files/api.html#service-lookup">META-INF/services</a> directory and it
will be available only on Windows.</p>
</li>
<li><p>Another possibility that does not require new module, but which executes
a code on startup (which may have performance implications) is to use <code>methodvalue</code>
attribute. Register your instance in layer using <code>your-Object.instance</code> file
as described at
<a href="@org-openide-util@/org/openide/util/doc-files/api.html#ido-methodvalue">services
</a> documentation and in your factory method either return the instance
your want or <code>null</code> depending on result of <a href="@org-openide-util@/org/openide/util/BaseUtilities.html#isWindows()">
Utilities.isWindows()</a> call.</p>
</li>
<li>
<p>
In some cases, the interface for which you will register an implementation permits a
no-operation semantics. For example, <code>InstalledFileLocator.locate(...)</code> can
return a valid <code>File</code>, or null. You could always register an
<code>InstalledFileLocator</code> instance yet disable it on non-Windows platforms
(always returning null).
</p>
</li>
</ul>
</usecase>
<usecase id="lookup-extension-point" name="How shall I write an extension point for my module?" >
<p>
<em><b>Q:</b>
I have more modules one of them providing the core functionality and
few more that wish to extend it. What is the right way to do it?
How does the Netbeans platform declare such extension point?
</em>
</p>
<p>
Start with declaring an extension interface in your
core module and put it into the module's <em>public packages</em>. Imagine
for example that the core module is in JAR file <code>org-my-netbeans-coremodule.jar</code>
and already contains in manifests line like
<code>OpenIDE-Module: org.my.netbeans.coremodule/1</code> and wants
to display various tips of the day provided by other modules and thus defines:
</p><pre>
<span class="java-keywords">package</span> <span class="java-identifier">org</span><span class="java-operators">.</span><span class="java-identifier">my</span><span class="java-operators">.</span><span class="java-identifier">netbeans</span><span class="java-operators">.</span><span class="java-identifier">coremodule</span><span class="java-operators">;</span>
<span class="java-keywords">public</span> <span class="java-keywords">interface</span> <span class="java-identifier">TipsOfTheDayProvider</span> <span class="java-operators">{</span>
<span class="java-keywords">public</span> <span class="java-identifier">String</span> <span class="java-layer-method">provideTipOfTheDay</span> <span class="java-operators">(</span><span class="java-operators">)</span><span class="java-operators">;</span>
<span class="java-operators">}</span>
</pre><p>
And in its manifest adds line
<code>OpenIDE-Module-Public-Packages: org.my.netbeans.coremodule.*</code>
to specify that this package contains exported API and shall be
accessible to other modules.
</p>
<p>
When the core module is about to display the tip of the day it can ask
the system for all registered instances of the <code>TipsOfTheDayProvider</code>,
randomly select one of them:
</p><pre>
<span class="java-keywords">import</span> <span class="java-identifier">java</span><span class="java-operators">.</span><span class="java-identifier">util</span><span class="java-operators">.</span><span class="java-identifier">Collection</span><span class="java-operators">;</span>
<span class="java-keywords">import</span> <span class="java-identifier">java</span><span class="java-operators">.</span><span class="java-identifier">util</span><span class="java-operators">.</span><span class="java-identifier">Collections</span><span class="java-operators">;</span>
<span class="java-keywords">import</span> <span class="java-identifier">org</span><span class="java-operators">.</span><span class="java-identifier">openide</span><span class="java-operators">.</span><span class="java-identifier">util</span><span class="java-operators">.</span><span class="java-identifier">Lookup</span><span class="java-operators">;</span>
<a href="@TOP@org/openide/util/Lookup.Result.html"><span class="java-identifier">Lookup</span><span class="java-operators">.</span><span class="java-identifier">Result</span></a> <span class="java-identifier">result</span> <span class="java-operators">=</span> <a href="@TOP@org/openide/util/Lookup.html"><span class="java-identifier">Lookup</span></a><span class="java-operators">.</span><span class="java-layer-method">getDefault</span> <span class="java-operators">(</span><span class="java-operators">)</span><span class="java-operators">.</span><span class="java-layer-method">lookup</span> <span class="java-operators">(</span><span class="java-keywords">new</span> <a href="@TOP@org/openide/util/Lookup.Template.html"><span class="java-identifier">Lookup</span><span class="java-operators">.</span><span class="java-layer-method">Template</span></a> <span class="java-operators">(</span><span class="java-identifier">TipsOfTheDayProvider</span><span class="java-operators">.</span><span class="java-keywords">class</span><span class="java-operators">)</span><span class="java-operators">)</span><span class="java-operators">;</span>
<span class="java-identifier">Collection</span> <span class="java-identifier">c</span> <span class="java-operators">=</span> <span class="java-identifier">result</span><span class="java-operators">.</span><a href="@TOP@org/openide/util/Lookup.Result.html#allInstances()"><span class="java-layer-method">allInstances</span></a> <span class="java-operators">(</span><span class="java-operators">)</span><span class="java-operators">;</span>
<span class="java-identifier">Collections</span><span class="java-operators">.</span><span class="java-layer-method">shuffle</span> <span class="java-operators">(</span><span class="java-identifier">c</span><span class="java-operators">)</span><span class="java-operators">;</span>
<span class="java-identifier">TipsOfTheDayProvider</span> <span class="java-identifier">selected</span> <span class="java-operators">=</span> <span class="java-operators">(</span><span class="java-identifier">TipsOfTheDayProvider</span><span class="java-operators">)</span><span class="java-identifier">c</span><span class="java-operators">.</span><span class="java-layer-method">iterator</span> <span class="java-operators">(</span><span class="java-operators">)</span><span class="java-operators">.</span><span class="java-layer-method">next</span> <span class="java-operators">(</span><span class="java-operators">)</span><span class="java-operators">;</span>
</pre><p>
and then display the tip. Simple, trivial, just by the usage of
<a href="@TOP@org/openide/util/Lookup.html">Lookup</a> interface once
creates a registry that other modules can enhance. But such enhancing
of course requires work on the other side. Each module that would like
to register its <code>TipsOfTheDayProvider</code> needs to depend on the
core module - add
<code>OpenIDE-Module-Module-Dependencies: org.my.netbeans.coremodule/1</code>
into its manifest and write a class with its own implementation of the
provider:</p><pre>
<span class="java-keywords">package</span> <span class="java-identifier">org</span><span class="java-operators">.</span><span class="java-identifier">my</span><span class="java-operators">.</span><span class="java-identifier">netbeans</span><span class="java-operators">.</span><span class="java-identifier">extramodule</span><span class="java-operators">;</span>
<span class="java-keywords">class</span> <span class="java-identifier">ExtraTip</span> <span class="java-keywords">implements</span> <span class="java-identifier">TipsOfTheDayProvider</span> <span class="java-operators">{</span>
<span class="java-keywords">public</span> <span class="java-identifier">String</span> <span class="java-layer-method">provideTipOfTheDay</span> <span class="java-operators">(</span><span class="java-operators">)</span> <span class="java-operators">{</span>
<span class="java-keywords">return</span> <span class="java-string-literal">"Do you know that in order to write extension point you should use Lookup?"</span><span class="java-operators">;</span>
<span class="java-operators">}</span>
<span class="java-operators">}</span>
</pre><p>
Then, the only necessary thing is to register such class by using the
J2SE standard <api name="ProviderRegistrationMechanism" type="import"
category="standard"
group="java" /> into plain text file
<code>META-INF/services/org.my.netbeans.coremodule.TipsOfTheDayProvider</code>
in the module JAR containing just one line: </p><pre>
org.my.netbeans.extramodule.ExtraTip
</pre><p>
and your modules are now ready to communicate
using your own <em>extension point</em>.
</p>
</usecase>
</answer>
<answer id="arch-what">
<p>
Described in the <a href="@TOP@/architecture-summary.html#answer-arch-overall">overall</a> answer.
</p>
</answer>
<answer id="compat-standards">
<p>
The default lookup registration follows the JDK's
<api name="ProviderRegistrationMechanism" type="import"
category="standard" url="https://docs.oracle.com/javase/1.5.0/docs/guide/jar/jar.html#Provider%20Configuration%20File"
group="java"
/> <!-- ref to 1.5.0 as 1.3 is not reacheable -->
but enhances it to also support the
<api
name="ProviderRegistrationRemoval" type="export"
category="devel" url="@org-openide-util@/org/openide/util/doc-files/api.html#service-lookup"
group="java"
/>.
</p>
</answer>
<answer id="compat-version">
<p>
This module has no settings.
</p>
</answer>
<answer id="dep-jre">
<p>
Currently JRE 1.5 is needed.
</p>
</answer>
<answer id="dep-jrejdk">
<p>
JRE is enough.
</p>
</answer>
<answer id="deploy-jar">
<p>
<api category="devel" group="java.io.File" name="FileLocation"type="export" >
the JAR file is located in platform cluster under <code>lib/org-openide-util-lookup.jar</code>
</api>.
</p>
</answer>
<answer id="deploy-shared">
<p>
Module is on real java classpath and as such it has to be in the shared directory.
</p>
</answer>
<answer id="exec-process">
<p>
No external processes executed.
</p>
</answer>
<answer id="exec-property">
<ul>
<li>
<api type="export" group="systemproperty" name="org.openide.util.Lookup" category="devel">
checked by the initialization of the
<a href="@TOP@/org/openide/util/Lookup.html#getDefault()">Lookup.getDefault()</a>
and can
contain name of a class that extends <code>org.openide.util.Lookup</code> and
has public constructor, that should be instantiated and returned from
<a href="@TOP@/org/openide/util/Lookup.html#getDefault()">Lookup.getDefault()</a>
the class will be loaded by
<a href="@JDK@@JDKMODULE_JAVA_BASE@/java/lang/Thread.html#getContextClassLoader()">
Thread.currentThread().getContextClassLoader()</a>
classloader the first time <code>Lookup.getDefault</code> is invoked.
<p>
The property can also contain value <code>"-"</code> which means to completely
disable the lookup instantiation and return <a href="@TOP@/org/openide/util/Lookup.html#EMPTY">Lookup.EMPTY</a>
from <a href="@TOP@/org/openide/util/Lookup.html#getDefault()">Lookup.getDefault()</a>.
</p>
If the property is unspecified, the default <code>MetaInfServicesLookup</code>
is constructed for <code>Thread.currentThread().getContextclassLoader()</code>
that implements the <a href="architecture-summary.html#answer-compat-standards">JDK's standard</a>. If, by
a chance an instance of
<a href="@TOP@/org/openide/util/Lookup.Provider.html">Lookup.Provider</a>
is found
in there, its lookup is returned as result. Otherwise the <code>MetaInfServicesLookup</code>
is the result of <a href="@TOP@/org/openide/util/Lookup.html#getDefault()">Lookup.getDefault()</a>.
</api>
</li>
<li>
<api type="export" group="systemproperty" name="org.openide.util.Lookup.paths" category="devel">
Sometimes it may be useful for the Lookup to contains objects from
some system file system folder. This can be done with
<code>org.openide.util.Lookup.paths=Folder1:Folder2:Folder3</code>.
If this property is set prior to first call to
<a href="@TOP@/org/openide/util/Lookup.html#getDefault()">Lookup.getDefault()</a>,
it is split into pieces (separator is <code>':'</code>) and individual
parts are then used to construct <code>Lookups.forPath("Folder1")</code>,
etc. All these lookups then become part of the
<a href="@TOP@/org/openide/util/Lookup.html#getDefault()">Lookup.getDefault()</a>
one. This property works since version7.24
</api>
</li>
</ul>
</answer>
<answer id="exec-reflection">
<p>
<api category="devel" group="java" name="Lookups.metaInfServices"type="export"url="@TOP@/org/openide/util/lookup/Lookups.html#metaInfServices(java.lang.ClassLoader)">
calls constructor of registered classes using reflection
</api>.
<api category="friend" group="java" name="Lookup.resetDefaultLookup"type="export">
There is a static private method <code>Lookup.resetDefaultLookup</code> that
is called by NbJUnit's <code>MockServices</code> to properly reset default
lookup and fire changes to all registred listeners.
</api>.
</p>
</answer>
<answer id="exec-threading">
<p>
Everything is synchronous, except pluggable use of <code>java.util.concurrent.Executor</code>
that allows to make calls asynchronous. The default implementation only delivers
changes from <a href="@TOP@/org/openide/util/lookup/Lookups.html#metaInfServices(java.lang.ClassLoader)">metaInfServices</a>
lookup in asynchronous thread.
</p>
</answer>
<answer id="format-clipboard">
<p>
Not used.
</p>
</answer>
<answer id="format-dnd">
<p>
The same as for clipboard.
</p>
</answer>
<answer id="format-types">
<p>
No.
</p>
</answer>
<answer id="lookup-lookup">
<ul>
<li><api name="LookupInitializationLookup" category="devel" group="lookup"type="export"url="#systemproperty-org.openide.util.Lookup">
during
initialization of the <a href="@TOP@/org/openide/util/Lookup.html#getDefault()">Lookup.getDefault()</a>
the <a href="@TOP@/org/openide/util/Lookup.Provider.html">Lookup.Provider</a>
is being searched</api>.
</li>
<li><api name="LookupSharedClassObject" category="devel" group="lookup"type="export">
singleton subclasses of <a href="@org-openide-util-ui@/org/openide/util/SharedClassObject.html">SharedClassObject</a>
are searched for using Lookup.
</api>.</li>
<li><api name="LookupClassLoader" category="devel" group="lookup"type="export">
Nearly all resource looking functions and reflective code
uses <a href="@JDK@@JDKMODULE_JAVA_BASE@/java/lang/ClassLoader.html">ClassLoader</a>
obtained from <a href="@TOP@/org/openide/util/Lookup.html#getDefault()">Lookup.getDefault()</a>
for loading system wide resources.
</api>.</li>
<!-- <questionid="perf-limit"when="init"> Arethereanyhard-codedorpracticallimitsinthenumberorsizeof elementsyourcodecanhandle? </question>
-->
<answer id="perf-limit">
<p>
The default implementation of the <code>MetaInfServicesLookup</code> just
keeps hashmap between queried classes and their implementations. The amount
of memory is linear to amount of registered classes, but of course we
are not counting the memory occupied by the instances which the lookup
creates, that can be arbitrary.
</p>
</answer>
<!-- <questionid="perf-mem"when="final"> Howmuchmemorydoesyourcomponentconsume?Estimate witharelationtothenumberofwindows,etc. </question>
-->
<answer id="perf-mem">
<p>
There are no big data structures. The amount of memory occupied by
instances of <a href="@TOP@/org/openide/util/lookup/AbstractLookup.html">AbstractLookup</a>
is measured by unit tests.
</p>
</answer>
<!-- <questionid="perf-menus"when="final"> Doesyourmoduleusedynamicallyupdatedcontextmenus,or context-sensitiveactionswithcomplicatedandslowenablementlogic? <hint> Ifyoudoalotoftrickswhenaddingactionstoregularorcontextmenus,youcansignificantly slowdowndisplayofthemenu,evenwhentheuserisnotusingyouraction.Payattentionto actionsyouaddtothemainmenubar,andtocontextmenusofforeignnodesorcomponents.If theactionisconditionallyenabled,orchangesitsdisplaydynamically,youneedtocheckthe impactonperformance.Insomecasesitmaybemoreappropriatetomakeasimpleactionthatis alwaysenabledbutdoesmoredetailedchecksinadialogifitisactuallyrun. </hint> </question>
-->
<answer id="perf-menus">
<p>
There are no menus.
</p>
</answer>
Die Informationen auf dieser Webseite wurden
nach bestem Wissen sorgfältig zusammengestellt. Es wird jedoch weder Vollständigkeit, noch Richtigkeit,
noch Qualität der bereit gestellten Informationen zugesichert.
Bemerkung:
Die farbliche Syntaxdarstellung und die Messung sind noch experimentell.