The following class diagram is surprisingly complex due to the complicated strategy that Java uses to implement extendable stream handlers and content handlers.

Figure 52.1 Class Diagram, java.net (URL and associated classes)

A bundle that can implement a new URL scheme should register a service object under the URLStreamHandlerService interface with the OSGi Framework. This interface contains public versions of the java.net.URLStreamHandler class methods, so that these methods can be called by the proxy (the object that is actually registered with the Java run-time).

The OSGi Framework implementation must make this service object available to the underlying java.net implementation. This must be supported by the OSGi Framework implementation because the java.net.URL.setStreamHandlerFactory method can only be called once, making it impossible to use by bundles that come and go.

Bundles that can convert a content-typed stream should register a service object under the name java.net.ContentHandler. These objects should be made available by the OSGi Framework to the java.net.URLConnection class.

When a URL is used with a previously unused scheme, it must query the registered java.net.URLStreamHandlerFactory object (that should have been registered by the OSGi Framework). The OSGi Framework must then search the service registry for services that are registered under URLStreamHandlerService and that match the requested scheme.

If one or more service objects are found, a proxy object must be constructed. A proxy object is necessary because the service object that provides the implementation of the java.net.URLStreamHandler object can become unregistered and Java does not provide a mechanism to withdraw a java.net.URLStreamHandler object once it is returned from a java.net.URLStreamHandlerFactory object.

Once the proxy is created, it must track the service registry for registrations and unregistrations of services matching its associated scheme. The proxy must be associated with the service that matches the scheme and has the highest ranking (first in ranking order) at any moment in time. If a proxy is associated with a URL Stream Handler Service, it must change the associated handler to a newly registered service when that service has a higher ranking.

The proxy object must forward all method requests to the associated URL Stream Handler Service until this service object becomes unregistered.

Once a proxy is created, it cannot be withdrawn because it is cached by the Java run-time. However, service objects can be withdrawn and it is possible for a proxy to exist without an associated URLStreamHandlerService/java.net.ContentHandler object.

In this case, the proxy must handle subsequent requests until another appropriate service is registered. When this happens, the proxy class must handle the error.

In the case of a URL Stream Handler proxy, it must throw a java.net.MalformedURLException exception if the signature of a method allows throwing this exception. Otherwise, a java.lang.IllegalStateException exception is thrown.

In the case of a Content Handler proxy, it must return InputStream to the data.

Bundles must ensure that their URLStreamHandlerService or java.net.ContentHandler service objects throw these exceptions also when they have become unregistered.

Proxies for Content Handler services operate slightly differently from URL Stream Handler Service proxies. In the case that null is returned from the registered ContentHandlerFactory object, the factory will not get another chance to provide a ContentHandler object for that content-type. Thus, if there is no built-in handler, nor a registered handler for this content-type, a ContentHandler proxy must be constructed that returns the InputStream object from the URLConnection object as the content object until a handler is registered.

Implementations of Java provide a number of sub-classes of java.net.URLStreamHandler classes that can handle protocols like HTTP, FTP, NEWS etc. Most Java implementations provide a mechanism to add new handlers that can be found on the class path through class name construction.

If a registered java.net.URLStreamHandlerFactory object returns null for a built-in handler (or one that is available through the class name construction mechanism), it will never be called again for that specific scheme because the Java implementation will use its built-in handler or uses the class name construction.

As a result, even though it is not forbidden for URL Handlers Service implementations to override built-in handlers, it is not possible to guarantee that a registered URLStreamHandlerService object will be used when it is overriding a built-in handler. For consistency reasons, it is therefore recommended to never override built-in handlers.

The Content Handler Factory is implemented using a similar technique and has therefore the same problems.

To facilitate the discovery of built-in handlers that are available through the name construction, the method described in the next section must be used by the Framework before any handlers are searched for in the service registry.

If the system properties java.protocol.handler.pkgs or java.content.handler.pkgs are defined, they must be used to locate built-in handlers. Each property must be defined as a list of package names that are separated by a vertical line ('|' \u007C) and that are searched in the left-to-right order (the names must not end in a full stop ('.' \u002E). For example:

org.osgi.impl.handlers | com.acme.url

The package names are the prefixes that are put in front of a scheme or content type to form a class name that can handle the scheme or content-type.

A URL Stream Handler name for a scheme is formed by appending the string ".Handler" to the scheme name. Using the packages in the previous example, the rsh scheme handler class is searched by the following names:

org.osgi.impl.handlers.rsh.Handler
com.acme.url.rsh.Handler

MIME type names contain the solidus ('/' \u002F) character and can contain other characters that must not be part of a Java class name. A MIME type name must be processed as follows before it can be converted to a class name:

The Java run-time specific packages should be listed in the appropriate properties so that implementations of the URL Stream Handler Factory and Content Handler Factory can be made aware of these packages.

Implementations of java.net.URLStreamHandler class cannot be registered in the service registry for use by the proxy because the methods of the URLStreamHandler class are protected and thus not available to the proxy implementation. Also, the URLStreamHandler class checks that only the URLStreamHandler object that was returned from the URLStreamHandlerFactory object can invoke the setURL method. This means that URLStreamHandler objects in the service registry would be unable to invoke the setURL method. Invoking this method is necessary when implementing the parseURL method.

Therefore, the URLStreamHandlerService and URLStreamHandlerSetter interfaces were created. The URLStreamHandlerService interface provides public versions of the URLStreamHandler methods, except that the setURL method is missing and the parseURL method has a new first argument of type URLStreamHandlerSetter. In general, sub-classes of the URLStreamHandler class can be converted to URLStreamHandlerService classes with minimal code changes. Apart from making the relevant methods public, the parseURL method needs to be changed to invoke the setURL method on the URLStreamHandlerSetter object that the URLStreamHandlerService object was passed, rather then the setURL method of URLStreamHandler class.

Figure 52.2 Proxy Issues

To aid in the conversion of URLStreamHandler implementation classes, the AbstractURLStreamHandlerService has been provided. Apart from making the relevant methods public, the AbstractURLStreamHandlerService stores the URLStreamHandlerSetter object in a private variable. To make the setURL method work properly, it overrides the setURL method to invoke the setURL method on the saved URLStreamHandlerSetter object rather then the URLStreamHandler.setURL method. This means that a subclass of URLStreamHandler should be changed to become a sub-class of the AbstractURLStreamHandlerService class and be recompiled.

Normally, the parseURL method will have the following form:

class URLStreamHandlerImpl {
    ...
    protected URLStreamHandlerSetter realHandler;
    ... 
    public void parseURL(
        URLStreamHandlerSetter realHandler,
            URL u, String spec, int start, int limit) {
            this.realHandler = realHandler;
            parseURL(u, spec, start, limit);
    }
    protected void setURL(URL u,
        String protocol, String host,
        int port, String authority,
        String userInfo, String path,
        String query,String ref) {
            realHandler.setURL(u, protocol, host, 
                port, authority, userInfo, path,
                query, ref);
    }
    ...
}

The URLStreamHandler.parseURL method will call the setURL method which must be invoked on the proxy rather than this. That is why the setURL method is overridden to delegate to the URLStreamHandlerSetter object in realHandler as opposed to super.

Java 1.5 introduced a new method on the URLStreamHandler class: URLConnection openConnection(URL,Proxy). Adding this method to the URL Stream Handler service poses the following problems:

Therefore, scheme providers can optionally implement the openConnection(URL,Proxy) method as a public method. If the scheme provider implements this method, then the framework must call it (using reflection). If this method is not implemented in the URL Stream Handler service an Unsupported Operation Exception must be thrown.

Framework implementations should be careful not to create unwanted dependencies on Java 1.5. This will require two different implementation classes for the URLStreamHandler class that is used to proxy the URL Stream Handler services.