深入学习java源码之FileDescriptor.valid()与FileDescriptor.sync()

深入学习java源码之FileDescriptor.valid()与FileDescriptor.sync()

文件描述符类的实例用作表示打开文件，开放套接字或其他字节源或信宿的底层机器特定结构的不透明句柄。 文件描述符的主要实际用途是创建一个FileInputStream或FileOutputStream来包含它。  应用程序不应创建自己的文件描述符。  字段

Modifier and TypeField and Descriptionstatic FileDescriptorerr

标准错误流的句柄。

static FileDescriptorin

标准输入流的句柄。

static FileDescriptorout

标准输出流的句柄。

方法

Modifier and TypeMethod and Descriptionvoidsync()

强制所有系统缓冲区与底层设备同步。

booleanvalid()

测试此文件描述符对象是否有效。

 

java源码

package java.io;

import java.util.ArrayList;
import java.util.List;

public final class FileDescriptor {

    private int fd;

    private long handle;

    private Closeable parent;
    private List otherParents;
    private boolean closed;

    public /**/ FileDescriptor() {
        fd = -1;
        handle = -1;
    }

    static {
        initIDs();
    }

    // Set up JavaIOFileDescriptorAccess in SharedSecrets
    static {
        sun.misc.SharedSecrets.setJavaIOFileDescriptorAccess(
            new sun.misc.JavaIOFileDescriptorAccess() {
                public void set(FileDescriptor obj, int fd) {
                    obj.fd = fd;
                }

                public int get(FileDescriptor obj) {
                    return obj.fd;
                }

                public void setHandle(FileDescriptor obj, long handle) {
                    obj.handle = handle;
                }

                public long getHandle(FileDescriptor obj) {
                    return obj.handle;
                }
            }
        );
    }

    public static final FileDescriptor in = standardStream(0);

    public static final FileDescriptor out = standardStream(1);

    public static final FileDescriptor err = standardStream(2);

    public boolean valid() {
        return ((handle != -1) || (fd != -1));
    }

    public native void sync() throws SyncFailedException;

    /* This routine initializes JNI field offsets for the class */
    private static native void initIDs();

    private static native long set(int d);

    private static FileDescriptor standardStream(int fd) {
        FileDescriptor desc = new FileDescriptor();
        desc.handle = set(fd);
        return desc;
    }

    synchronized void attach(Closeable c) {
        if (parent == null) {
            // first caller gets to do this
            parent = c;
        } else if (otherParents == null) {
            otherParents = new ArrayList();
            otherParents.add(parent);
            otherParents.add(c);
        } else {
            otherParents.add(c);
        }
    }

    @SuppressWarnings("try")
    synchronized void closeAll(Closeable releaser) throws IOException {
        if (!closed) {
            closed = true;
            IOException ioe = null;
            try (Closeable c = releaser) {
                if (otherParents != null) {
                    for (Closeable referent : otherParents) {
                        try {
                            referent.close();
                        } catch(IOException x) {
                            if (ioe == null) {
                                ioe = x;
                            } else {
                                ioe.addSuppressed(x);
                            }
                        }
                    }
                }
            } catch(IOException ex) {
                /*
                 * If releaser close() throws IOException
                 * add other exceptions as suppressed.
                 */
                if (ioe != null)
                    ex.addSuppressed(ioe);
                ioe = ex;
            } finally {
                if (ioe != null)
                    throw ioe;
            }
        }
    }
}