深入學習java源碼之FileDescriptor.valid()與FileDescriptor.sync()

深入學習java源碼之FileDescriptor.valid()與FileDescriptor.sync()

檔案描述符類的執行個體用作表示打開檔案，開放套接字或其他位元組源或信宿的底層機器特定結構的不透明句柄。 檔案描述符的主要實際用途是建立一個FileInputStream或FileOutputStream來包含它。 

應用程式不應建立自己的檔案描述符。 

字段

Modifier and Type	Field and Description
static FileDescriptor	err 标準錯誤流的句柄。
static FileDescriptor	in 标準輸入流的句柄。
static FileDescriptor	out 标準輸出流的句柄。

方法

Modifier and Type	Method and Description
void	sync() 強制所有系統緩沖區與底層裝置同步。
boolean	valid() 測試此檔案描述符對象是否有效。

java源碼

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<Closeable> 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;
            }
        }
    }
}