AsynchronousChannelProvider.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * accompanied this code).
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package java.nio.channels.spi;

import java.io.IOException;
import java.nio.channels.AsynchronousChannelGroup;
import java.nio.channels.AsynchronousServerSocketChannel;
import java.nio.channels.AsynchronousSocketChannel;
import java.nio.channels.IllegalChannelGroupException;
import java.nio.channels.ShutdownChannelGroupException;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Iterator;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.ThreadFactory;
import sun.nio.ch.DefaultAsynchronousChannelProvider;

/**
 * Service-provider class for asynchronous channels.
 *
 * <p> An asynchronous channel 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> All of the methods in this class are safe for use by multiple concurrent
 * threads.  </p>
 *
 * @since 1.7
 */
// 异步通道组和异步Socket通道的工厂
public abstract class AsynchronousChannelProvider {
    
    /**
     * Initializes a new instance of this class.
     *
     * @throws SecurityException If a security manager has been installed and it denies
     *                           {@link RuntimePermission}{@code ("asynchronousChannelProvider")}
     */
    protected AsynchronousChannelProvider() {
        this(checkPermission());
    }
    
    private AsynchronousChannelProvider(Void ignore) {
    }
    
    
    /**
     * Returns the system-wide default asynchronous channel 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
     * {@code java.nio.channels.spi.AsynchronousChannelProvider} 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.AsynchronousChannelProvider} 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 AsynchronousChannel provider
     */
    // 返回系统默认的异步通道组工厂
    public static AsynchronousChannelProvider provider() {
        return ProviderHolder.provider;
    }
    
    /**
     * Constructs a new asynchronous channel group with a fixed thread pool.
     *
     * @param nThreads      The number of threads in the pool
     * @param threadFactory The factory to use when creating new threads
     *
     * @return A new asynchronous channel group
     *
     * @throws IllegalArgumentException If {@code nThreads <= 0}
     * @throws IOException              If an I/O error occurs
     * @see AsynchronousChannelGroup#withFixedThreadPool
     */
    // 返回一个带有固定容量线程池的异步通道组，线程池容量为nThreads
    public abstract AsynchronousChannelGroup openAsynchronousChannelGroup(int nThreads, ThreadFactory threadFactory) throws IOException;
    
    /**
     * Constructs a new asynchronous channel group with the given thread pool.
     *
     * @param executor    The thread pool
     * @param initialSize A value {@code >=0} or a negative value for implementation specific default
     *
     * @return A new asynchronous channel group
     *
     * @throws IOException If an I/O error occurs
     * @see AsynchronousChannelGroup#withCachedThreadPool
     */
    // 返回一个包含指定线程池的异步通道组，线程池初始容量为initialSize(具体值还需要进一步计算)
    public abstract AsynchronousChannelGroup openAsynchronousChannelGroup(ExecutorService executor, int initialSize) throws IOException;
    
    /**
     * Opens an asynchronous socket channel.
     *
     * @param group The group to which the channel is bound, or {@code null} to bind to the default group
     *
     * @return The new channel
     *
     * @throws IllegalChannelGroupException  If the provider that created the group differs from this provider
     * @throws ShutdownChannelGroupException The group is shutdown
     * @throws IOException                   If an I/O error occurs
     */
    // 打开一个异步Socket通道，group是该通道关联的异步通道组
    public abstract AsynchronousSocketChannel openAsynchronousSocketChannel(AsynchronousChannelGroup group) throws IOException;
    
    /**
     * Opens an asynchronous server-socket channel.
     *
     * @param group The group to which the channel is bound, or {@code null} to
     *              bind to the default group
     *
     * @return The new channel
     *
     * @throws IllegalChannelGroupException  If the provider that created the group differs from this provider
     * @throws ShutdownChannelGroupException The group is shutdown
     * @throws IOException                   If an I/O error occurs
     */
    // 打开一个异步ServerSocket通道，group是该通道关联的异步通道组
    public abstract AsynchronousServerSocketChannel openAsynchronousServerSocketChannel(AsynchronousChannelGroup group) throws IOException;
    
    // 权限检查
    private static Void checkPermission() {
        SecurityManager sm = System.getSecurityManager();
        if(sm != null) {
            sm.checkPermission(new RuntimePermission("asynchronousChannelProvider"));
        }
        return null;
    }
    
    
    // 异步通道工厂引用
    private static class ProviderHolder {
        static final AsynchronousChannelProvider provider = load();
        
        private static AsynchronousChannelProvider load() {
            return AccessController.doPrivileged(new PrivilegedAction<>() {
                public AsynchronousChannelProvider run() {
                    AsynchronousChannelProvider p;
                    
                    // 尝试从用户定义的属性中创建异步通道工厂，默认为null
                    p = loadProviderFromProperty();
                    if(p != null) {
                        return p;
                    }
                    
                    // 尝试从用户注册的服务中创建异步通道工厂，默认为null
                    p = loadProviderAsService();
                    if(p != null) {
                        return p;
                    }
                    
                    // 返回系统默认的异步通道工厂
                    return DefaultAsynchronousChannelProvider.create();
                }
            });
        }
        
        // 尝试从用户定义的属性中创建异步通道工厂，默认为null
        private static AsynchronousChannelProvider loadProviderFromProperty() {
            String provider = System.getProperty("java.nio.channels.spi.AsynchronousChannelProvider");
            if(provider == null) {
                return null;
            }
            
            try {
                @SuppressWarnings("deprecation")
                Object tmp = Class.forName(provider, true, ClassLoader.getSystemClassLoader()).newInstance();
                return (AsynchronousChannelProvider) tmp;
            } catch(ClassNotFoundException | IllegalAccessException | InstantiationException | SecurityException x) {
                throw new ServiceConfigurationError(null, x);
            }
        }
        
        // 尝试从用户注册的服务中创建异步通道工厂，默认为null
        private static AsynchronousChannelProvider loadProviderAsService() {
            ServiceLoader<AsynchronousChannelProvider> serviceLoader = ServiceLoader.load(AsynchronousChannelProvider.class, ClassLoader.getSystemClassLoader());
            Iterator<AsynchronousChannelProvider> iterator = serviceLoader.iterator();
            for(; ; ) {
                try {
                    return (iterator.hasNext()) ? iterator.next() : null;
                } catch(ServiceConfigurationError sce) {
                    if(sce.getCause() instanceof SecurityException) {
                        // Ignore the security exception, try the next provider
                        continue;
                    }
                    throw sce;
                }
            }
        }
    }
    
}