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Service.javaAPI DocJava SE 5 API11955Fri Aug 26 14:55:00 BST 2005com.sun.jmx.remote.util

Service

public final class Service extends Object
EXTRACTED FROM sun.misc.Service A simple service-provider lookup mechanism. A service is a well-known set of intjavax.management.remoteerfaces and (usually abstract) classes. A service provider is a specific implementation of a service. The classes in a provider typically implement the interfaces and subclass the classes defined in the service itself. Service providers may be installed in an implementation of the Java platform in the form of extensions, that is, jar files placed into any of the usual extension directories. Providers may also be made available by adding them to the applet or application class path or by some other platform-specific means.

In this lookup mechanism a service is represented by an interface or an abstract class. (A concrete class may be used, but this is not recommended.) A provider of a given service contains one or more concrete classes that extend this service class with data and code specific to the provider. This provider class will typically not be the entire provider itself but rather a proxy that contains enough information to decide whether the provider is able to satisfy a particular request together with code that can create the actual provider on demand. The details of provider classes tend to be highly service-specific; no single class or interface could possibly unify them, so no such class has been defined. The only requirement enforced here is that provider classes must have a zero-argument constructor so that they may be instantiated during lookup.

A service provider identifies itself by placing a provider-configuration file in the resource directory META-INF/services. The file's name should consist of the fully-qualified name of the abstract service class. The file should contain a list of fully-qualified concrete provider-class names, one per line. Space and tab characters surrounding each name, as well as blank lines, are ignored. The comment character is '#' (0x23); on each line all characters following the first comment character are ignored. The file must be encoded in UTF-8.

If a particular concrete provider class is named in more than one configuration file, or is named in the same configuration file more than once, then the duplicates will be ignored. The configuration file naming a particular provider need not be in the same jar file or other distribution unit as the provider itself. The provider must be accessible from the same class loader that was initially queried to locate the configuration file; note that this is not necessarily the class loader that found the file.

Example: Suppose we have a service class named java.io.spi.CharCodec. It has two abstract methods: 

public abstract CharEncoder getEncoder(String encodingName);
public abstract CharDecoder getDecoder(String encodingName);
Each method returns an appropriate object or null if it cannot translate the given encoding. Typical CharCodec providers will support more than one encoding.

If sun.io.StandardCodec is a provider of the CharCodec service then its jar file would contain the file META-INF/services/java.io.spi.CharCodec. This file would contain the single line: 

sun.io.StandardCodec # Standard codecs for the platform
To locate an encoder for a given encoding name, the internal I/O code would do something like this: 
CharEncoder getEncoder(String encodingName) {
Iterator ps = Service.providers(CharCodec.class);
while (ps.hasNext()) {
CharCodec cc = (CharCodec)ps.next();
CharEncoder ce = cc.getEncoder(encodingName);
if (ce != null)
return ce;
}
return null;
}
The provider-lookup mechanism always executes in the security context of the caller. Trusted system code should typically invoke the methods in this class from within a privileged security context.

Fields Summary
private static final String
prefix
Constructors Summary
private Service()
Methods Summary
private static voidfail(java.lang.Class service, java.lang.String msg, java.lang.Throwable cause)

	IllegalArgumentException sce
	    = new IllegalArgumentException(service.getName() + ": " + msg);
	
	throw (IllegalArgumentException) EnvHelp.initCause(sce, cause);
private static voidfail(java.lang.Class service, java.lang.String msg)

	throw new IllegalArgumentException(service.getName() + ": " + msg);
private static voidfail(java.lang.Class service, java.net.URL u, int line, java.lang.String msg)

	fail(service, u + ":" + line + ": " + msg);
private static java.util.Iteratorparse(java.lang.Class service, java.net.URL u, java.util.Set returned)
Parse the content of the given URL as a provider-configuration file.

param
service The service class for which providers are being sought; used to construct error detail strings
param
url The URL naming the configuration file to be parsed
param
returned A Set containing the names of provider classes that have already been returned. This set will be updated to contain the names that will be yielded from the returned Iterator.
return
A (possibly empty) Iterator that will yield the provider-class names in the given configuration file that are not yet members of the returned set
throws
IllegalArgumentException If an I/O error occurs while reading from the given URL, or if a configuration-file format error is detected

	InputStream in = null;
	BufferedReader r = null;
	ArrayList names = new ArrayList();
	try {
	    in = u.openStream();
	    r = new BufferedReader(new InputStreamReader(in, "utf-8"));
	    int lc = 1;
	    while ((lc = parseLine(service, u, r, lc, names, returned)) >= 0);
	} catch (IOException x) {
	    fail(service, ": " + x);
	} finally {
	    try {
		if (r != null) r.close();
		if (in != null) in.close();
	    } catch (IOException y) {
		fail(service, ": " + y);
	    }
	}
	return names.iterator();
private static intparseLine(java.lang.Class service, java.net.URL u, java.io.BufferedReader r, int lc, java.util.List names, java.util.Set returned)
Parse a single line from the given configuration file, adding the name on the line to both the names list and the returned set iff the name is not already a member of the returned set.

	String ln = r.readLine();
	if (ln == null) {
	    return -1;
	}
	int ci = ln.indexOf('#");
	if (ci >= 0) ln = ln.substring(0, ci);
	ln = ln.trim();
	int n = ln.length();
	if (n != 0) {
	    if ((ln.indexOf(' ") >= 0) || (ln.indexOf('\t") >= 0))
		fail(service, u, lc, "Illegal configuration-file syntax");
	    if (!Character.isJavaIdentifierStart(ln.charAt(0)))
		fail(service, u, lc, "Illegal provider-class name: " + ln);
	    for (int i = 1; i < n; i++) {
		char c = ln.charAt(i);
		if (!Character.isJavaIdentifierPart(c) && (c != '."))
		    fail(service, u, lc, "Illegal provider-class name: " + ln);
	    }
	    if (!returned.contains(ln)) {
		names.add(ln);
		returned.add(ln);
	    }
	}
	return lc + 1;
public static java.util.Iteratorproviders(java.lang.Class service, java.lang.ClassLoader loader)
Locates and incrementally instantiates the available providers of a given service using the given class loader.

This method transforms the name of the given service class into a provider-configuration filename as described above and then uses the getResources method of the given class loader to find all available files with that name. These files are then read and parsed to produce a list of provider-class names. The iterator that is returned uses the given class loader to lookup and then instantiate each element of the list.

Because it is possible for extensions to be installed into a running Java virtual machine, this method may return different results each time it is invoked.

param
service The service's abstract service class
param
loader The class loader to be used to load provider-configuration files and instantiate provider classes, or null if the system class loader (or, failing that the bootstrap class loader) is to be used
return
An Iterator that yields provider objects for the given service, in some arbitrary order. The iterator will throw a IllegalArgumentException if a provider-configuration file violates the specified format or if a provider class cannot be found and instantiated.
throws
IllegalArgumentException If a provider-configuration file violates the specified format or names a provider class that cannot be found and instantiated

	return new LazyIterator(service, loader);