Hello world! Writing the first OSGi application

HelloWorld
In general, learning a new technology, programmers are accustomed to first development of a Hello World application, which seems to be an "industrial standard." OK, let's get started, it's not difficult to develop a simple OSGi application: Create a plug-in project, File > New > Project, select Plug-in development > Plug-in PROJEC T
Figure 1. New Plug-in Project
In establishing the project's first wizard, fill in the project name: Osgi.test.helloworld, using the default engineering path. Note the choice of the target platform, because our project is a generic OSGi bundle, so choose Equinox.

Figure 2. Fill in Project name and select target platform


In the next wizard interface, fill in some of the plug-in information you need (note that the plug-in concept in Eclipse is basically similar to the concept of bundle in OSGi), where you need to fill in the OSGi provider (vendor) and classpath. If you don't have a special design, you can generally ignore these two fields. The last part about Activator is that if it's not a fragment bundle it needs to be filled in, which doesn't seem to be necessary unless your bundle to implement the framework's event monitor yourself. Therefore, it is recommended that you use the default settings, as shown in Figure 3:


Figure 3. Use default settings

Activator: This is the program entry that was first invoked when the bundle started, equivalent to the main function in the Java module. The difference is that main needs to be invoked through the command line, and OSGi's Activator is passively accepting calls from the OSGi framework, which starts only when the message is received.

Best Practice: Do not write too many startup code in the Activator, otherwise it will affect the bundle boot speed and the associated service startup can be placed in the service listener. The last step is not to use any of the templates, so check out the default options, click Finish, as shown in Figure 4: 

Figure 4. Check out the default options


Complete, the basic plug-in view as shown in Figure 5,eclipse will create the same path in the project name Java Package, which contains the Activator class, and the configuration information of the plug-in are also placed in MANIFEST. MF file, we have quite a lot of work to do in the future. &NBSP

Figure 5. Basic Plug-in View


Edit Activator.java, enter the Hello World statement with the following code: &NBSP

Listing 1. Edit Activator.java

Package Osgi.test.helloworld; Import Org.osgi.framework.BundleActivator; Import Org.osgi.framework.BundleContext; public class Activator implements Bundleactivator { /* * (Non-javadoc) * @see Org.osgi.framework.BundleActivator * #start (org.osgi.framework.BundleContext) */public void Start (Bundlecontext context) throws Exception { System.out.println ("Hello World"); } * * (non-javadoc) * @see org.osgi.framework.BundleActivator * #stop ( Org.osgi.framework.BundleContext) * /public void Stop (Bundlecontext context) throws Exception { } }

We can see that each Activator actually implements the Bundleactivator interface, which enables Activator to accept the invocation of the framework. After the frame is started, each bundle is invoked to invoke each bundle's Activator.

Note: The bundle Activator must contain parameterless constructors so that the framework can use Class.newinstance () to reflect the Activator instance of the constructed bundle.

Here we fill in the Hello World string we want to output in the Start method. So, how can we start this bundle? Executive: Select Run > Open run Dialog, go to the running menu, right-click in the OSGi framework and select New OSGi runtime Environment, as shown in:

Figure 6. New OSGi Runtime Environment

In the right side of the running Environment dialog box, enter the name of the running environment, start level and dependent plug-ins, because we do not need other third-party plug-ins, so we only need to check the system Org.eclipse.osgi Plug-ins, if not selected this plugin, Hello World Will not run. As shown in Figure 7, only when you click on the Validate Bundles button, and the hint is no problem, it shows that your operating environment is basically OK.

Figure 7. Select Org.eclipse.osgi Plugin

Dependent plug-in selection:

Figure 8. Choice of dependent Plug-ins

OK, if your running environment is OK, click Run.

Figure 9. Running an OSGi project

Congratulations on your success.

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OSGi Console

The OSGi console is still relatively new to developers who are accustomed to developing common Java applications. In general, through the OSGi console, you can manage the lifecycle of all bundle in the system, as well as view the system environment, start, stop the entire frame, set the startup level, and so on. As shown in Figure 10, you can view the status of all bundle by typing SS:
Figure 10. View the status of all bundle

The main console commands are listed below:
Table 1. Equinox OSGi's main console command table

Category	Command	Meaning
Control framework	Launch	Start frame
Shutdown	Stop frame
Close	Close, exit frame
Exit	Exit immediately, equivalent to System.exit
Init	Uninstall all bundle (provided it is already shutdown)
SetProp	Set properties that are performed at run time
Control bundle	Install	Installation
Uninstall	Unloading
Start	Start
Stop	Stop it
Refresh	Refresh
Update	Update
Show status	Status	Show installed bundle and registered services
Ss	Show the simple state of all bundle
Services	Show details of the registration service
Packages	Show the status of import and export packages
Bundles	Show the status of all installed bundles
Headers	Displays the bundles header information, namely the MANIFEST. What's in MF
Log	Display LOG Entry Information
Other	Exec	Executes a command (blocking state) in another process
Fork	Unlike EXEC, it doesn't cause obstruction.
Gc	Promote garbage collection
Getprop	Get a property, or a property
Control startup level	Sl	Get the start level information for a bundle or the entire frame
Setfwsl	Set the start level of a frame
Setbsl	Set the start level of the bundle
Setibsl	Sets the start level of the initialization bundle

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MANIFEST. Mf

MANIFEST. MF may appear in any Jar package that includes main class information, typically located in the Meta-inf directory, so this file is not an OSGi-specific thing, but simply adds some attributes, which just keeps the OSGi environment and the common Java environment consistent and easy to deploy on older systems. Table 2 lists the important attributes in this file and their implications:
Table 2. MANIFEST. MF file Properties

Attribute name	Meaning
Bundle-activator	The Bundle launcher
Bundle-symbolicname	Name, typically using a name similar to the JAVA package path
Bundle-version	Version, note that different versions of the same name bundle can be deployed online simultaneously
Export-package	Exported package declarations, other bundle can be referenced directly
Import-package	Imported Package
Eclipse-lazystart	Is it only started when it is referenced?
Require-bundle	Full reliance on bundle, not recommended
Bundle-classpath	The class path of this bundle can contain some other resource paths
Bundle-requiredexecutionenvironment	The required execution environment for this bundle, such as JDK version declarations

Category: Notes, OSGI