NetBeans Architecture Answers for Lookup API module

General Information

Question (arch-what): What is this project good for?
Answer:

Described in the overall answer.

Question (arch-overall): Describe the overall architecture.
Answer:

This module defines the Lookup 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 LookupAPI - allows the discovery and the LookupSPI - simplifies creation and registration of own lookup objects.

Question (arch-usecases): Describe the main use cases of the new API. Who will use it under what circumstances? What kind of code would typically need to be written to use the module?
Answer: There is a great introduction to Lookup and its usage in its javadoc. For details on this topic, together with code samples, see chapter 4, of NetBeans Platform for Beginners 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:

Lookup faq:

How to specify that a service in Lookup should be available only on Windows?

Q: 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).

How can I specify (in the xml, or programmatically) that this service should only be added to the Lookup if the platform is Windows? >

In general there are three ways to achieve this.

• It is possible to write a specific module and enable it only on windows. See os specific modules documentation. Then you can put a registration of your instance into your module's META-INF/services directory and it will be available only on Windows.

• Another possibility that does not require new module, but which executes a code on startup (which may have performance implications) is to use methodvalue attribute. Register your instance in layer using your-Object.instance file as described at services documentation and in your factory method either return the instance your want or null depending on result of Utilities.isWindows() call.

• In some cases, the interface for which you will register an implementation permits a no-operation semantics. For example, InstalledFileLocator.locate(...) can return a valid File, or null. You could always register an InstalledFileLocator instance yet disable it on non-Windows platforms (always returning null).

How shall I write an extension point for my module?

Q: 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?

Start with declaring an extension interface in your core module and put it into the module's public packages. Imagine for example that the core module is in JAR file org-my-netbeans-coremodule.jar and already contains in manifests line like OpenIDE-Module: org.my.netbeans.coremodule/1 and wants to display various tips of the day provided by other modules and thus defines:

                         
package org.my.netbeans.coremodule;

public interface TipsOfTheDayProvider {
    public String provideTipOfTheDay ();
}

                    

And in its manifest adds line OpenIDE-Module-Public-Packages: org.my.netbeans.coremodule.* to specify that this package contains exported API and shall be accessible to other modules.

When the core module is about to display the tip of the day it can ask the system for all registered instances of the TipsOfTheDayProvider, randomly select one of them:

                        
                        import java.util.Collection;
import java.util.Collections;
import org.openide.util.Lookup;

Lookup.Result result = Lookup.getDefault ().lookup (new Lookup.Template (TipsOfTheDayProvider.class));
Collection c = result.allInstances ();
Collections.shuffle (c);
TipsOfTheDayProvider selected = (TipsOfTheDayProvider)c.iterator ().next ();

                    

and then display the tip. Simple, trivial, just by the usage of Lookup 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 TipsOfTheDayProvider needs to depend on the core module - add OpenIDE-Module-Module-Dependencies: org.my.netbeans.coremodule/1 into its manifest and write a class with its own implementation of the provider:

                        
                        package org.my.netbeans.extramodule;

class ExtraTip implements TipsOfTheDayProvider {
    public String provideTipOfTheDay () {
        return "Do you know that in order to write extension point you should use Lookup?";
    }
}

                    

Then, the only necessary thing is to register such class by using the J2SE standard ProviderRegistrationMechanism into plain text file META-INF/services/org.my.netbeans.coremodule.TipsOfTheDayProvider in the module JAR containing just one line:

org.my.netbeans.extramodule.ExtraTip

and your modules are now ready to communicate using your own extension point.

Question (arch-time): What are the time estimates of the work?
Answer:

The module has been around since 1997 and is continously being improved from time to time.

Question (arch-quality): How will the quality of your code be tested and how are future regressions going to be prevented?
Answer:

There is a lot of unit tests in version control system.

Question (arch-where): Where one can find sources for your module?
Answer:

The sources for the module are in the Apache Git repositories or in the GitHub repositories.

Project and platform dependencies

Question (dep-nb): What other NetBeans projects and modules does this one depend on?
Answer:

N/A

The default answer to this question is:


These modules are required in project.xml:

Question (dep-non-nb): What other projects outside NetBeans does this one depend on?
WARNING: Question with id="dep-non-nb" has not been answered!
Question (dep-platform): On which platforms does your module run? Does it run in the same way on each?
Answer:

Platform independent.

Question (dep-jre): Which version of JRE do you need (1.2, 1.3, 1.4, etc.)?
Answer:

Currently JRE 1.5 is needed.

Question (dep-jrejdk): Do you require the JDK or is the JRE enough?
Answer:

JRE is enough.

Deployment

Question (deploy-jar): Do you deploy just module JAR file(s) or other files as well?
Answer:

FileLocation - the JAR file is located in platform cluster under lib/org-openide-util-lookup.jar .

Question (deploy-nbm): Can you deploy an NBM via the Update Center?
WARNING: Question with id="deploy-nbm" has not been answered!
Question (deploy-shared): Do you need to be installed in the shared location only, or in the user directory only, or can your module be installed anywhere?
Answer:

Module is on real java classpath and as such it has to be in the shared directory.

Question (deploy-packages): Are packages of your module made inaccessible by not declaring them public?
WARNING: Question with id="deploy-packages" has not been answered!
Question (deploy-dependencies): What do other modules need to do to declare a dependency on this one, in addition to or instead of the normal module dependency declaration (e.g. tokens to require)?
Answer:

Nothing.

Compatibility with environment

Question (compat-i18n): Is your module correctly internationalized?
WARNING: Question with id="compat-i18n" has not been answered!
Question (compat-standards): Does the module implement or define any standards? Is the implementation exact or does it deviate somehow?
Answer:

The default lookup registration follows the JDK's ProviderRegistrationMechanism but enhances it to also support the ProviderRegistrationRemoval.

Question (compat-version): Can your module coexist with earlier and future versions of itself? Can you correctly read all old settings? Will future versions be able to read your current settings? Can you read or politely ignore settings stored by a future version?
Answer:

This module has no settings.

Question (compat-deprecation): How the introduction of your project influences functionality provided by previous version of the product?
Answer:

XXX no answer for compat-deprecation

Access to resources

Question (resources-file): Does your module use java.io.File directly?
Answer:

No.

Question (resources-layer): Does your module provide own layer? Does it create any files or folders in it? What it is trying to communicate by that and with which components?
Answer:

No.

Question (resources-read): Does your module read any resources from layers? For what purpose?
Answer:

No.

Question (resources-mask): Does your module mask/hide/override any resources provided by other modules in their layers?
Answer:

No.

Question (resources-preferences): Does your module uses preferences via Preferences API? Does your module use NbPreferences or or regular JDK Preferences ? Does it read, write or both ? Does it share preferences with other modules ? If so, then why ?
Answer:

XXX no answer for resources-preferences

Lookup of components

Question (lookup-lookup): Does your module use org.openide.util.Lookup or any similar technology to find any components to communicate with? Which ones?
Answer:
• LookupInitializationLookup - during initialization of the Lookup.getDefault() the Lookup.Provider is being searched.
• LookupSharedClassObject - singleton subclasses of SharedClassObject are searched for using Lookup. .
• LookupClassLoader - Nearly all resource looking functions and reflective code uses ClassLoader obtained from Lookup.getDefault() for loading system wide resources. .
Question (lookup-register): Do you register anything into lookup for other code to find?
Answer:

No.

Question (lookup-remove): Do you remove entries of other modules from lookup?
Answer:

No.

Execution Environment

Question (exec-property): Is execution of your code influenced by any environment or Java system (System.getProperty) property? On a similar note, is there something interesting that you pass to java.util.logging.Logger? Or do you observe what others log?
Answer:
• org.openide.util.Lookup - checked by the initialization of the Lookup.getDefault() and can contain name of a class that extends org.openide.util.Lookup and has public constructor, that should be instantiated and returned from Lookup.getDefault() the class will be loaded by Thread.currentThread().getContextClassLoader() classloader the first time Lookup.getDefault is invoked.
  The property can also contain value "-" which means to completely disable the lookup instantiation and return Lookup.EMPTY from Lookup.getDefault().
  If the property is unspecified, the default MetaInfServicesLookup is constructed for Thread.currentThread().getContextclassLoader() that implements the JDK's standard. If, by a chance an instance of Lookup.Provider is found in there, its lookup is returned as result. Otherwise the MetaInfServicesLookup is the result of Lookup.getDefault().
• org.openide.util.Lookup.paths - Sometimes it may be useful for the Lookup to contains objects from some system file system folder. This can be done with org.openide.util.Lookup.paths=Folder1:Folder2:Folder3. If this property is set prior to first call to Lookup.getDefault(), it is split into pieces (separator is ':') and individual parts are then used to construct Lookups.forPath("Folder1"), etc. All these lookups then become part of the Lookup.getDefault() one. This property works since version 7.24
Question (exec-component): Is execution of your code influenced by any (string) property of any of your components?
WARNING: Question with id="exec-component" has not been answered!
Question (exec-ant-tasks): Do you define or register any ant tasks that other can use?
Answer:

No.

Question (exec-classloader): Does your code create its own class loader(s)?
Answer:

No, we do not create own classloader.

Question (exec-reflection): Does your code use Java Reflection to execute other code?
Answer:

Lookups.metaInfServices - calls constructor of registered classes using reflection . Lookup.resetDefaultLookup - There is a static private method Lookup.resetDefaultLookup that is called by NbJUnit's MockServices to properly reset default lookup and fire changes to all registred listeners. .

Question (exec-privateaccess): Are you aware of any other parts of the system calling some of your methods by reflection?
Answer:

No.

Question (exec-process): Do you execute an external process from your module? How do you ensure that the result is the same on different platforms? Do you parse output? Do you depend on result code?
Answer:

No external processes executed.

Question (exec-introspection): Does your module use any kind of runtime type information (instanceof, work with java.lang.Class, etc.)?
WARNING: Question with id="exec-introspection" has not been answered!
Question (exec-threading): What threading models, if any, does your module adhere to? How the project behaves with respect to threading?
Answer:

Everything is synchronous, except pluggable use of java.util.concurrent.Executor that allows to make calls asynchronous. The default implementation only delivers changes from metaInfServices lookup in asynchronous thread.

Question (security-policy): Does your functionality require modifications to the standard policy file?
Answer:

No security permissions manipulated.

Question (security-grant): Does your code grant additional rights to some other code?
Answer:

No security permitions manipulated.

Format of files and protocols

Question (format-types): Which protocols and file formats (if any) does your module read or write on disk, or transmit or receive over the network? Do you generate an ant build script? Can it be edited and modified?
Answer:

No.

Question (format-dnd): Which protocols (if any) does your code understand during Drag & Drop?
Answer:

The same as for clipboard.

Question (format-clipboard): Which data flavors (if any) does your code read from or insert to the clipboard (by access to clipboard on means calling methods on java.awt.datatransfer.Transferable?
Answer:

Not used.

Performance and Scalability

Question (perf-startup): Does your module run any code on startup?
Answer:

No.

Question (perf-exit): Does your module run any code on exit?
Answer:

Nothing.

Question (perf-scale): Which external criteria influence the performance of your program (size of file in editor, number of files in menu, in source directory, etc.) and how well your code scales?
Answer:

Lookup code scales linearily.

Question (perf-limit): Are there any hard-coded or practical limits in the number or size of elements your code can handle?
Answer:

The default implementation of the MetaInfServicesLookup 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.

Question (perf-mem): How much memory does your component consume? Estimate with a relation to the number of windows, etc.
Answer:

There are no big data structures. The amount of memory occupied by instances of AbstractLookup is measured by unit tests.

Question (perf-wakeup): Does any piece of your code wake up periodically and do something even when the system is otherwise idle (no user interaction)?
Answer:

No.

Question (perf-progress): Does your module execute any long-running tasks?
Answer:

No.

Question (perf-huge_dialogs): Does your module contain any dialogs or wizards with a large number of GUI controls such as combo boxes, lists, trees, or text areas?
Answer:

No.

Question (perf-menus): Does your module use dynamically updated context menus, or context-sensitive actions with complicated and slow enablement logic?
Answer:

There are no menus.

Question (perf-spi): How the performance of the plugged in code will be enforced?
Answer:

No enforcing is done.