FileCleanable.java

/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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.
 *
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package java.io;

import jdk.internal.misc.JavaIOFileDescriptorAccess;
import jdk.internal.misc.SharedSecrets;
import jdk.internal.ref.CleanerFactory;
import jdk.internal.ref.PhantomCleanable;

import java.lang.ref.Cleaner;

/**
 * Cleanable for a FileDescriptor when it becomes phantom reachable.
 * For regular fds on Unix and regular handles on Windows
 * register a cleanup if fd != -1 or handle != -1.
 * <p>
 * Subclassed from {@code PhantomCleanable} so that {@code clear} can be
 * called to disable the cleanup when the handle is closed by any means other
 * than calling {@link FileDescriptor#close}.
 * Otherwise, it might incorrectly close the handle after it has been reused.
 */

/*
 *    Reference
 *        │
 * PhantomReference   Cleanable
 *        └──────┬────────┘
 *        PhantomCleanable
 *               │
 *        FileCleanable
 *
 * 文件描述符（虚引用）清理器，此类的属性既是虚引用，又是清理器
 */
final class FileCleanable extends PhantomCleanable<FileDescriptor> {
    // Access to FileDescriptor private fields; avoids making fd and handle package private
    private static final JavaIOFileDescriptorAccess fdAccess = SharedSecrets.getJavaIOFileDescriptorAccess();
    
    // FileDescriptor的fd和handle
    private final int fd;
    private final long handle;
    
    /**
     * Constructor for a phantom cleanable reference.
     *
     * @param obj     the object to monitor
     * @param cleaner the cleaner
     * @param fd      file descriptor to close
     * @param handle  handle to close
     */
    private FileCleanable(FileDescriptor obj, Cleaner cleaner, int fd, long handle) {
        super(obj, cleaner);
        this.fd = fd;
        this.handle = handle;
    }
    
    /**
     * Register a Cleanable with the FileDescriptor if the FileDescriptor is non-null and valid.
     *
     * @param fdo the FileDescriptor; may be null
     *
     * @implNote A exception (OutOfMemoryException) will leave the FileDescriptor having allocated resources and leak the fd/handle.
     */
    // 注册文件描述符fdo到清理器
    static void register(FileDescriptor fdo) {
        if(fdo != null && fdo.valid()) {
            int fd = fdAccess.get(fdo);
            long handle = fdAccess.getHandle(fdo);
            /*
             * 向文件描述符（虚引用）清理器注册一个追踪对象：文件描述符对象
             * 当文件描述符对象被回收，可以在ReferenceQueue中收到通知
             */
            PhantomCleanable<FileDescriptor> cleanable = new FileCleanable(fdo, CleanerFactory.cleaner(), fd, handle);
            
            // 在文件描述符对象中关联此清理器
            fdo.registerCleanup(cleanable);
        }
    }
    
    /**
     * Unregister a Cleanable from the FileDescriptor.
     *
     * @param fdo the FileDescriptor; may be null
     */
    // 取消注册
    static void unregister(FileDescriptor fdo) {
        if(fdo != null) {
            // 置空关联的文件描述符（虚引用）清理器
            fdo.unregisterCleanup();
        }
    }
    
    /**
     * Close the native handle or fd.
     */
    // 关闭本地的handle和fd。CleanerImpl.run()==>清理器clean()-->清理器performCleanup()
    @Override
    protected void performCleanup() {
        try {
            cleanupClose0(fd, handle);
        } catch(IOException ioe) {
            throw new UncheckedIOException("close", ioe);
        }
    }
    
    /*
     * Raw close of the file fd and/or handle.
     * Used only for last chance cleanup.
     */
    private static native void cleanupClose0(int fd, long handle) throws IOException;
}