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.
*
*/
public final class Service {
private static final String prefix = "META-INF/services/";
private Service() { }
private static void fail(Class service, String msg, Throwable cause)
throws IllegalArgumentException
{
IllegalArgumentException sce
= new IllegalArgumentException(service.getName() + ": " + msg);
throw (IllegalArgumentException) EnvHelp.initCause(sce, cause);
}
private static void fail(Class service, String msg)
throws IllegalArgumentException
{
throw new IllegalArgumentException(service.getName() + ": " + msg);
}
private static void fail(Class service, URL u, int line, String msg)
throws IllegalArgumentException
{
fail(service, u + ":" + line + ": " + msg);
}
/**
* 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.
*/
private static int parseLine(Class service, URL u, BufferedReader r, int lc,
List names, Set returned)
throws IOException, IllegalArgumentException
{
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;
}
/**
* 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
*/
private static Iterator parse(Class service, URL u, Set returned)
throws IllegalArgumentException
{
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 inner class implementing fully-lazy provider lookup
*/
private static class LazyIterator implements Iterator {
Class service;
ClassLoader loader;
Enumeration configs = null;
Iterator pending = null;
Set returned = new TreeSet();
String nextName = null;
private LazyIterator(Class service, ClassLoader loader) {
this.service = service;
this.loader = loader;
}
public boolean hasNext() throws IllegalArgumentException {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
String fullName = prefix + service.getName();
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
configs = loader.getResources(fullName);
} catch (IOException x) {
fail(service, ": " + x);
}
}
while ((pending == null) || !pending.hasNext()) {
if (!configs.hasMoreElements()) {
return false;
}
pending = parse(service, (URL)configs.nextElement(), returned);
}
nextName = (String)pending.next();
return true;
}
public Object next() throws IllegalArgumentException {
if (!hasNext()) {
throw new NoSuchElementException();
}
String cn = nextName;
nextName = null;
try {
return Class.forName(cn, true, loader).newInstance();
} catch (ClassNotFoundException x) {
fail(service,
"Provider " + cn + " not found");
} catch (Exception x) {
fail(service,
"Provider " + cn + " could not be instantiated: " + x,
x);
}
return null; /* This cannot happen */
}
public void remove() {
throw new UnsupportedOperationException();
}
}
/**
* 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 * */ public static Iterator providers(Class service, ClassLoader loader) throws IllegalArgumentException { return new LazyIterator(service, loader); } }