FileDescriptor是文件描述符，可以被用来表示开放文件，开放套接字等，FileDescriptor可以被看成某个文件，但无法对该文件进行操作，需要新创建FileDescriptor对应的FileOutputStream再对文件进行操作。
FileDescriptor示例代码：
public class FileDescriptorTest {
private static final String FileName = "file.txt";
private static final String OutText = "Hi FileDescriptor";
public static void main(String[] args) {
testWrite();
testRead();
testStandFD() ;
//System.out.println(OutText);
}
/**
* FileDescriptor.out 的测试程序
* 该程序的效果 等价于 System.out.println(OutText);
*/
private static void testStandFD() {
// 创建FileDescriptor.out 对应的PrintStream
PrintStream out = new PrintStream(new FileOutputStream(FileDescriptor.out));
// 在屏幕上输出“Hi FileDescriptor”
out.println(OutText);
out.close();
}
/**
* FileDescriptor写入示例程序
* (01) 为了说明，"通过文件名创建FileOutputStream"与“通过文件描述符创建FileOutputStream”对象是等效的
* (02) 该程序会在“该源文件”所在目录新建文件"file.txt"，并且文件内容是"Aa"。
*/
private static void testWrite() {
try {
// 新建文件“file.txt”对应的FileOutputStream对象
FileOutputStream out1 = new FileOutputStream(FileName);
// 获取文件“file.txt”对应的“文件描述符”
FileDescriptor fdout = out1.getFD();
// 根据“文件描述符”创建“FileOutputStream”对象
FileOutputStream out2 = new FileOutputStream(fdout);
out1.write('A'); // 通过out1向“file.txt”中写入'A'
out2.write('a'); // 通过out2向“file.txt”中写入'A'
if (fdout!=null)
System.out.printf("fdout(%s) is %s\n",fdout, fdout.valid());
out1.close();
out2.close();
} catch(IOException e) {
e.printStackTrace();
}
}
/**
* FileDescriptor读取示例程序
*
* 为了说明，"通过文件名创建FileInputStream"与“通过文件描述符创建FileInputStream”对象是等效的
*/
private static void testRead() {
try {
// 新建文件“file.txt”对应的FileInputStream对象
FileInputStream in1 = new FileInputStream(FileName);
// 获取文件“file.txt”对应的“文件描述符”
FileDescriptor fdin = in1.getFD();
// 根据“文件描述符”创建“FileInputStream”对象
FileInputStream in2 = new FileInputStream(fdin);
System.out.println("in1.read():"+(char)in1.read());
System.out.println("in2.read():"+(char)in2.read());
if (fdin!=null)
System.out.printf("fdin(%s) is %s\n", fdin, fdin.valid());
in1.close();
in2.close();
} catch(IOException e) {
e.printStackTrace();
}
}
}
运行结果：
fdout(java.io.FileDescriptor@1ea21c07) is true
in1.read():A
in2.read():a
fdin(java.io.FileDescriptor@24de1f47) is true
Hi FileDescriptor
in1.read():A
in2.read():a
fdin(java.io.FileDescriptor@24de1f47) is true
Hi FileDescriptor
基于JDK8的FileDescriptor的源码：
public final class FileDescriptor {
private int fd;
private long handle;
private Closeable parent;
private List<Closeable> otherParents;
private boolean closed;
/**
* Constructs an (invalid) FileDescriptor
* object.
*/
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;
}
}
);
}
/**
* A handle to the standard input stream. Usually, this file
* descriptor is not used directly, but rather via the input stream
* known as {@code System.in}.
*
* @see java.lang.System#in
*/
//标准输入流
public static final FileDescriptor in = standardStream(0);
/**
* A handle to the standard output stream. Usually, this file
* descriptor is not used directly, but rather via the output stream
* known as {@code System.out}.
* @see java.lang.System#out
*/
//标准输出流
public static final FileDescriptor out = standardStream(1);
/**
* A handle to the standard error stream. Usually, this file
* descriptor is not used directly, but rather via the output stream
* known as {@code System.err}.
*
* @see java.lang.System#err
*/
//标准错误
public static final FileDescriptor err = standardStream(2);
/**
* Tests if this file descriptor object is valid.
*
* @return {@code true} if the file descriptor object represents a
* valid, open file, socket, or other active I/O connection;
* {@code false} otherwise.
*/
//文件描述对象是否有效
public boolean valid() {
return ((handle != -1) || (fd != -1));
}
/**
* Force all system buffers to synchronize with the underlying
* device. This method returns after all modified data and
* attributes of this FileDescriptor have been written to the
* relevant device(s). In particular, if this FileDescriptor
* refers to a physical storage medium, such as a file in a file
* system, sync will not return until all in-memory modified copies
* of buffers associated with this FileDesecriptor have been
* written to the physical medium.
*
* sync is meant to be used by code that requires physical
* storage (such as a file) to be in a known state For
* example, a class that provided a simple transaction facility
* might use sync to ensure that all changes to a file caused
* by a given transaction were recorded on a storage medium.
*
* sync only affects buffers downstream of this FileDescriptor. If
* any in-memory buffering is being done by the application (for
* example, by a BufferedOutputStream object), those buffers must
* be flushed into the FileDescriptor (for example, by invoking
* OutputStream.flush) before that data will be affected by sync.
*
* @exception SyncFailedException
* Thrown when the buffers cannot be flushed,
* or because the system cannot guarantee that all the
* buffers have been synchronized with physical media.
* @since JDK1.1
*/
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;
}
/*
* Package private methods to track referents.
* If multiple streams point to the same FileDescriptor, we cycle
* through the list of all referents and call close()
*/
/**
* Attach a Closeable to this FD for tracking.
* parent reference is added to otherParents when
* needed to make closeAll simpler.
*/
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);
}
}
/**
* Cycle through all Closeables sharing this FD and call
* close() on each one.
*
* The caller closeable gets to call close0().
*/
@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;
}
}
}
}