1/* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 18package java.nio.channels.spi; 19 20import java.io.IOException; 21import java.nio.SelectorProviderImpl; 22import java.nio.channels.Channel; 23import java.nio.channels.DatagramChannel; 24import java.nio.channels.Pipe; 25import java.nio.channels.ServerSocketChannel; 26import java.nio.channels.SocketChannel; 27import java.util.ServiceLoader; 28 29/** 30 * {@code SelectorProvider} is an abstract base class that declares methods for 31 * providing instances of {@link DatagramChannel}, {@link Pipe}, 32 * {@link java.nio.channels.Selector} , {@link ServerSocketChannel}, and 33 * {@link SocketChannel}. All the methods of this class are thread-safe. 34 * 35 * <p>A provider instance can be retrieved through a system property or the 36 * configuration file in a jar file; if no provider is available that way then 37 * the system default provider is returned. 38 */ 39public abstract class SelectorProvider { 40 41 private static SelectorProvider provider = null; 42 43 /** 44 * Constructs a new {@code SelectorProvider}. 45 */ 46 protected SelectorProvider() { 47 } 48 49 /** 50 * Gets a provider instance by executing the following steps when called for 51 * the first time: 52 * <ul> 53 * <li> if the system property "java.nio.channels.spi.SelectorProvider" is 54 * set, the value of this property is the class name of the provider 55 * returned; </li> 56 * <li>if there is a provider-configuration file named 57 * "java.nio.channels.spi.SelectorProvider" in META-INF/services of a jar 58 * file valid in the system class loader, the first class name is the 59 * provider's class name; </li> 60 * <li> otherwise, a system default provider will be returned.</li> 61 * </ul> 62 * 63 * @return the provider. 64 */ 65 synchronized public static SelectorProvider provider() { 66 if (provider == null) { 67 provider = ServiceLoader.loadFromSystemProperty(SelectorProvider.class); 68 if (provider == null) { 69 provider = loadProviderByJar(); 70 } 71 if (provider == null) { 72 provider = new SelectorProviderImpl(); 73 } 74 } 75 return provider; 76 } 77 78 private static SelectorProvider loadProviderByJar() { 79 for (SelectorProvider provider : ServiceLoader.load(SelectorProvider.class)) { 80 return provider; 81 } 82 return null; 83 } 84 85 /** 86 * Creates a new open {@code DatagramChannel}. 87 * 88 * @return the new channel. 89 * @throws IOException 90 * if an I/O error occurs. 91 */ 92 public abstract DatagramChannel openDatagramChannel() throws IOException; 93 94 /** 95 * Creates a new {@code Pipe}. 96 * 97 * @return the new pipe. 98 * @throws IOException 99 * if an I/O error occurs. 100 */ 101 public abstract Pipe openPipe() throws IOException; 102 103 /** 104 * Creates a new selector. 105 * 106 * @return the new selector. 107 * @throws IOException 108 * if an I/O error occurs. 109 */ 110 public abstract AbstractSelector openSelector() throws IOException; 111 112 /** 113 * Creates a new open {@code ServerSocketChannel}. 114 * 115 * @return the new channel. 116 * @throws IOException 117 * if an I/O error occurs. 118 */ 119 public abstract ServerSocketChannel openServerSocketChannel() 120 throws IOException; 121 122 /** 123 * Create a new open {@code SocketChannel}. 124 * 125 * @return the new channel. 126 * @throws IOException 127 * if an I/O error occurs. 128 */ 129 public abstract SocketChannel openSocketChannel() throws IOException; 130 131 /** 132 * Returns the channel inherited from the process that created this VM. 133 * On Android, this method always returns null because stdin and stdout are 134 * never connected to a socket. 135 * 136 * @return the channel. 137 * @throws IOException 138 * if an I/O error occurs. 139 */ 140 public Channel inheritedChannel() throws IOException { 141 // Android never has stdin/stdout connected to a socket. 142 return null; 143 } 144} 145