Java tutorial
/* * Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.nio.channels.spi; import java.io.IOException; import java.net.ProtocolFamily; import java.nio.channels.*; import java.security.AccessController; import java.security.PrivilegedAction; import java.util.Iterator; import java.util.ServiceLoader; import java.util.ServiceConfigurationError; import sun.security.action.GetPropertyAction; /** * Service-provider class for selectors and selectable channels. * * <p> A selector provider is a concrete subclass of this class that has a * zero-argument constructor and implements the abstract methods specified * below. A given invocation of the Java virtual machine maintains a single * system-wide default provider instance, which is returned by the {@link * #provider() provider} method. The first invocation of that method will locate * the default provider as specified below. * * <p> The system-wide default provider is used by the static {@code open} * methods of the {@link java.nio.channels.DatagramChannel#open * DatagramChannel}, {@link java.nio.channels.Pipe#open Pipe}, {@link * java.nio.channels.Selector#open Selector}, {@link * java.nio.channels.ServerSocketChannel#open ServerSocketChannel}, and {@link * java.nio.channels.SocketChannel#open SocketChannel} classes. It is also * used by the {@link java.lang.System#inheritedChannel System.inheritedChannel()} * method. A program may make use of a provider other than the default provider * by instantiating that provider and then directly invoking the {@code open} * methods defined in this class. * * <p> All of the methods in this class are safe for use by multiple concurrent * threads. </p> * * * @author Mark Reinhold * @author JSR-51 Expert Group * @since 1.4 */ public abstract class SelectorProvider { private static final Object lock = new Object(); private static SelectorProvider provider = null; private static Void checkPermission() { SecurityManager sm = System.getSecurityManager(); if (sm != null) sm.checkPermission(new RuntimePermission("selectorProvider")); return null; } private SelectorProvider(Void ignore) { } /** * Initializes a new instance of this class. * * @throws SecurityException * If a security manager has been installed and it denies * {@link RuntimePermission}{@code ("selectorProvider")} */ protected SelectorProvider() { this(checkPermission()); } private static boolean loadProviderFromProperty() { String cn = System.getProperty("java.nio.channels.spi.SelectorProvider"); if (cn == null) return false; try { @SuppressWarnings("deprecation") Object tmp = Class.forName(cn, true, ClassLoader.getSystemClassLoader()).newInstance(); provider = (SelectorProvider) tmp; return true; } catch (ClassNotFoundException x) { throw new ServiceConfigurationError(null, x); } catch (IllegalAccessException x) { throw new ServiceConfigurationError(null, x); } catch (InstantiationException x) { throw new ServiceConfigurationError(null, x); } catch (SecurityException x) { throw new ServiceConfigurationError(null, x); } } private static boolean loadProviderAsService() { ServiceLoader<SelectorProvider> sl = ServiceLoader.load(SelectorProvider.class, ClassLoader.getSystemClassLoader()); Iterator<SelectorProvider> i = sl.iterator(); for (;;) { try { if (!i.hasNext()) return false; provider = i.next(); return true; } catch (ServiceConfigurationError sce) { if (sce.getCause() instanceof SecurityException) { // Ignore the security exception, try the next provider continue; } throw sce; } } } /** * Returns the system-wide default selector provider for this invocation of * the Java virtual machine. * * <p> The first invocation of this method locates the default provider * object as follows: </p> * * <ol> * * <li><p> If the system property * {@systemProperty java.nio.channels.spi.SelectorProvider} is defined * then it is taken to be the fully-qualified name of a concrete provider * class. The class is loaded and instantiated; if this process fails then * an unspecified error is thrown. </p></li> * * <li><p> If a provider class has been installed in a jar file that is * visible to the system class loader, and that jar file contains a * provider-configuration file named * {@code java.nio.channels.spi.SelectorProvider} in the resource * directory {@code META-INF/services}, then the first class name * specified in that file is taken. The class is loaded and * instantiated; if this process fails then an unspecified error is * thrown. </p></li> * * <li><p> Finally, if no provider has been specified by any of the above * means then the system-default provider class is instantiated and the * result is returned. </p></li> * * </ol> * * <p> Subsequent invocations of this method return the provider that was * returned by the first invocation. </p> * * @return The system-wide default selector provider */ public static SelectorProvider provider() { synchronized (lock) { if (provider != null) return provider; return AccessController.doPrivileged(new PrivilegedAction<>() { public SelectorProvider run() { if (loadProviderFromProperty()) return provider; if (loadProviderAsService()) return provider; provider = sun.nio.ch.DefaultSelectorProvider.create(); return provider; } }); } } /** * Opens a datagram channel. * * @return The new channel * * @throws IOException * If an I/O error occurs */ public abstract DatagramChannel openDatagramChannel() throws IOException; /** * Opens a datagram channel. * * @param family * The protocol family * * @return A new datagram channel * * @throws UnsupportedOperationException * If the specified protocol family is not supported * @throws IOException * If an I/O error occurs * * @since 1.7 */ public abstract DatagramChannel openDatagramChannel(ProtocolFamily family) throws IOException; /** * Opens a pipe. * * @return The new pipe * * @throws IOException * If an I/O error occurs */ public abstract Pipe openPipe() throws IOException; /** * Opens a selector. * * @return The new selector * * @throws IOException * If an I/O error occurs */ public abstract AbstractSelector openSelector() throws IOException; /** * Opens a server-socket channel. * * @return The new channel * * @throws IOException * If an I/O error occurs */ public abstract ServerSocketChannel openServerSocketChannel() throws IOException; /** * Opens a socket channel. * * @return The new channel * * @throws IOException * If an I/O error occurs */ public abstract SocketChannel openSocketChannel() throws IOException; /** * Returns the channel inherited from the entity that created this * Java virtual machine. * * <p> On many operating systems a process, such as a Java virtual * machine, can be started in a manner that allows the process to * inherit a channel from the entity that created the process. The * manner in which this is done is system dependent, as are the * possible entities to which the channel may be connected. For example, * on UNIX systems, the Internet services daemon (<i>inetd</i>) is used to * start programs to service requests when a request arrives on an * associated network port. In this example, the process that is started, * inherits a channel representing a network socket. * * <p> In cases where the inherited channel represents a network socket * then the {@link java.nio.channels.Channel Channel} type returned * by this method is determined as follows: * * <ul> * * <li><p> If the inherited channel represents a stream-oriented connected * socket then a {@link java.nio.channels.SocketChannel SocketChannel} is * returned. The socket channel is, at least initially, in blocking * mode, bound to a socket address, and connected to a peer. * </p></li> * * <li><p> If the inherited channel represents a stream-oriented listening * socket then a {@link java.nio.channels.ServerSocketChannel * ServerSocketChannel} is returned. The server-socket channel is, at * least initially, in blocking mode, and bound to a socket address. * </p></li> * * <li><p> If the inherited channel is a datagram-oriented socket * then a {@link java.nio.channels.DatagramChannel DatagramChannel} is * returned. The datagram channel is, at least initially, in blocking * mode, and bound to a socket address. * </p></li> * * </ul> * * <p> In addition to the network-oriented channels described, this method * may return other kinds of channels in the future. * * <p> The first invocation of this method creates the channel that is * returned. Subsequent invocations of this method return the same * channel. </p> * * @return The inherited channel, if any, otherwise {@code null}. * * @throws IOException * If an I/O error occurs * * @throws SecurityException * If a security manager has been installed and it denies * {@link RuntimePermission}{@code ("inheritedChannel")} * * @since 1.5 */ public Channel inheritedChannel() throws IOException { return null; } }