java.io
Class RandomAccessFile

java.lang.Object   
  java.io.RandomAccessFile

All Implemented Interfaces:

Closeable , DataInput , DataOutput

public class RandomAccessFile
extends Object   
implements DataOutput   , DataInput   , Closeable   

Instances of this class support both reading and writing to a random access file. A random access file behaves like a large array of bytes stored in the file system. There is a kind of cursor, or index into the implied array, called the file pointer; input operations read bytes starting at the file pointer and advance the file pointer past the bytes read. If the random access file is created in read/write mode, then output operations are also available; output operations write bytes starting at the file pointer and advance the file pointer past the bytes written. Output operations that write past the current end of the implied array cause the array to be extended. The file pointer can be read by the getFilePointer method and set by the seek method.

It is generally true of all the reading routines in this class that if end-of-file is reached before the desired number of bytes has been read, an EOFException (which is a kind of IOException) is thrown. If any byte cannot be read for any reason other than end-of-file, an IOException other than EOFException is thrown. In particular, an IOException may be thrown if the stream has been closed.

Since:

JDK1.0

Constructor Summary

RandomAccessFile (File file, String mode)
Creates a random access file stream to read from, and optionally to write to, the file specified by the File argument.

RandomAccessFile (String name, String mode)
Creates a random access file stream to read from, and optionally to write to, a file with the specified name.

Method Summary

void	close () Closes this random access file stream and releases any system resources associated with the stream.
FileChannel	getChannel () Returns the unique FileChannel object associated with this file.
FileDescriptor	getFD () Returns the opaque file descriptor object associated with this stream.
long	getFilePointer () Returns the current offset in this file.
long	length () Returns the length of this file.
int	read () Reads a byte of data from this file.
int	read (byte[] b) Reads up to b.length bytes of data from this file into an array of bytes.
int	read (byte[] b, int off, int len) Reads up to len bytes of data from this file into an array of bytes.
boolean	readBoolean () Reads a boolean from this file.
byte	readByte () Reads a signed eight-bit value from this file.
char	readChar () Reads a Unicode character from this file.
double	readDouble () Reads a double from this file.
float	readFloat () Reads a float from this file.
void	readFully (byte[] b) Reads b.length bytes from this file into the byte array, starting at the current file pointer.
void	readFully (byte[] b, int off, int len) Reads exactly len bytes from this file into the byte array, starting at the current file pointer.
int	readInt () Reads a signed 32-bit integer from this file.
String	readLine () Reads the next line of text from this file.
long	readLong () Reads a signed 64-bit integer from this file.
short	readShort () Reads a signed 16-bit number from this file.
int	readUnsignedByte () Reads an unsigned eight-bit number from this file.
int	readUnsignedShort () Reads an unsigned 16-bit number from this file.
String	readUTF () Reads in a string from this file.
void	seek (long pos) Sets the file-pointer offset, measured from the beginning of this file, at which the next read or write occurs.
void	setLength (long newLength) Sets the length of this file.
int	skipBytes (int n) Attempts to skip over n bytes of input discarding the skipped bytes.
void	write (byte[] b) Writes b.length bytes from the specified byte array to this file, starting at the current file pointer.
void	write (byte[] b, int off, int len) Writes len bytes from the specified byte array starting at offset off to this file.
void	write (int b) Writes the specified byte to this file.
void	writeBoolean (boolean v) Writes a boolean to the file as a one-byte value.
void	writeByte (int v) Writes a byte to the file as a one-byte value.
void	writeBytes (String s) Writes the string to the file as a sequence of bytes.
void	writeChar (int v) Writes a char to the file as a two-byte value, high byte first.
void	writeChars (String s) Writes a string to the file as a sequence of characters.
void	writeDouble (double v) Converts the double argument to a long using the doubleToLongBits method in class Double, and then writes that long value to the file as an eight-byte quantity, high byte first.
void	writeFloat (float v) Converts the float argument to an int using the floatToIntBits method in class Float, and then writes that int value to the file as a four-byte quantity, high byte first.
void	writeInt (int v) Writes an int to the file as four bytes, high byte first.
void	writeLong (long v) Writes a long to the file as eight bytes, high byte first.
void	writeShort (int v) Writes a short to the file as two bytes, high byte first.
void	writeUTF (String str) Writes a string to the file using modified UTF-8 encoding in a machine-independent manner.

Methods inherited from class java.lang.Object

clone , equals , finalize , getClass , hashCode , notify , notifyAll , toString , wait , wait , wait

Constructor Detail

RandomAccessFile

public RandomAccessFile(String    name,
                        String    mode)
                 throws FileNotFoundException   

Creates a random access file stream to read from, and optionally to write to, a file with the specified name. A new FileDescriptor object is created to represent the connection to the file.

The mode argument specifies the access mode with which the file is to be opened. The permitted values and their meanings are as specified for the RandomAccessFile(File,String) constructor.

If there is a security manager, its checkRead method is called with the name argument as its argument to see if read access to the file is allowed. If the mode allows writing, the security manager's checkWrite method is also called with the name argument as its argument to see if write access to the file is allowed.

Parameters:

name - the system-dependent filename

mode - the access mode

Throws:

IllegalArgumentException - if the mode argument is not equal to one of "r", "rw", "rws", or "rwd"

FileNotFoundException - if the mode is "r" but the given string does not denote an existing regular file, or if the mode begins with "rw" but the given string does not denote an existing, writable regular file and a new regular file of that name cannot be created, or if some other error occurs while opening or creating the file

SecurityException - if a security manager exists and its checkRead method denies read access to the file or the mode is "rw" and the security manager's checkWrite method denies write access to the file

See Also:

SecurityException , SecurityManager.checkRead(java.lang.String) , SecurityManager.checkWrite(java.lang.String)

RandomAccessFile

public RandomAccessFile(File    file,
                        String    mode)
                 throws FileNotFoundException   

Creates a random access file stream to read from, and optionally to write to, the file specified by the File argument. A new FileDescriptor object is created to represent this file connection.

The mode argument specifies the access mode in which the file is to be opened. The permitted values and their meanings are:

Value	Meaning
"r"	Open for reading only. Invoking any of the write methods of the resulting object will cause an IOException to be thrown.
"rw"	Open for reading and writing. If the file does not already exist then an attempt will be made to create it.
"rws"	Open for reading and writing, as with "rw", and also require that every update to the file's content or metadata be written synchronously to the underlying storage device.
"rwd"	Open for reading and writing, as with "rw", and also require that every update to the file's content be written synchronously to the underlying storage device.

The "rws" and "rwd" modes work much like the force(boolean) method of the FileChannel class, passing arguments of true and false, respectively, except that they always apply to every I/O operation and are therefore often more efficient. If the file resides on a local storage device then when an invocation of a method of this class returns it is guaranteed that all changes made to the file by that invocation will have been written to that device. This is useful for ensuring that critical information is not lost in the event of a system crash. If the file does not reside on a local device then no such guarantee is made.

The "rwd" mode can be used to reduce the number of I/O operations performed. Using "rwd" only requires updates to the file's content to be written to storage; using "rws" requires updates to both the file's content and its metadata to be written, which generally requires at least one more low-level I/O operation.

If there is a security manager, its checkRead method is called with the pathname of the file argument as its argument to see if read access to the file is allowed. If the mode allows writing, the security manager's checkWrite method is also called with the path argument to see if write access to the file is allowed.

Parameters:

file - the file object

mode - the access mode, as described above

Throws:

IllegalArgumentException - if the mode argument is not equal to one of "r", "rw", "rws", or "rwd"

FileNotFoundException - if the mode is "r" but the given file object does not denote an existing regular file, or if the mode begins with "rw" but the given file object does not denote an existing, writable regular file and a new regular file of that name cannot be created, or if some other error occurs while opening or creating the file

SecurityException - if a security manager exists and its checkRead method denies read access to the file or the mode is "rw" and the security manager's checkWrite method denies write access to the file

See Also:

SecurityManager.checkRead(java.lang.String) , SecurityManager.checkWrite(java.lang.String) , FileChannel.force(boolean)

Method Detail

getFD

public final FileDescriptor    getFD()
                           throws IOException   

Returns the opaque file descriptor object associated with this stream.

Returns:

the file descriptor object associated with this stream.

Throws:

IOException - if an I/O error occurs.

See Also:

FileDescriptor

getChannel

public final FileChannel    getChannel()

Returns the unique FileChannel object associated with this file.

The position of the returned channel will always be equal to this object's file-pointer offset as returned by the getFilePointer method. Changing this object's file-pointer offset, whether explicitly or by reading or writing bytes, will change the position of the channel, and vice versa. Changing the file's length via this object will change the length seen via the file channel, and vice versa.

Returns:

the file channel associated with this file

Since:

1.4

read

public int read()
         throws IOException   

Reads a byte of data from this file. The byte is returned as an integer in the range 0 to 255 (0x00-0x0ff). This method blocks if no input is yet available.

Although RandomAccessFile is not a subclass of InputStream, this method behaves in exactly the same way as the InputStream.read() method of InputStream.

Returns:

the next byte of data, or -1 if the end of the file has been reached.

Throws:

IOException - if an I/O error occurs. Not thrown if end-of-file has been reached.

read

public int read(byte[] b,
                int off,
                int len)
         throws IOException   

Reads up to len bytes of data from this file into an array of bytes. This method blocks until at least one byte of input is available.

Although RandomAccessFile is not a subclass of InputStream, this method behaves in exactly the same way as the InputStream.read(byte[], int, int) method of InputStream.

Parameters:

b - the buffer into which the data is read.

off - the start offset of the data.

len - the maximum number of bytes read.

Returns:

the total number of bytes read into the buffer, or -1 if there is no more data because the end of the file has been reached.

Throws:

IOException - if an I/O error occurs.

read

public int read(byte[] b)
         throws IOException   

Reads up to b.length bytes of data from this file into an array of bytes. This method blocks until at least one byte of input is available.

Although RandomAccessFile is not a subclass of InputStream, this method behaves in exactly the same way as the InputStream.read(byte[]) method of InputStream.

Parameters:

b - the buffer into which the data is read.

Returns:

the total number of bytes read into the buffer, or -1 if there is no more data because the end of this file has been reached.

Throws:

IOException - if an I/O error occurs.

readFully

public final void readFully(byte[] b)
                     throws IOException   

Reads b.length bytes from this file into the byte array, starting at the current file pointer. This method reads repeatedly from the file until the requested number of bytes are read. This method blocks until the requested number of bytes are read, the end of the stream is detected, or an exception is thrown.

Specified by:

readFully in interface DataInput

Parameters:

b - the buffer into which the data is read.

Throws:

EOFException - if this file reaches the end before reading all the bytes.

IOException - if an I/O error occurs.

readFully

public final void readFully(byte[] b,
                            int off,
                            int len)
                     throws IOException   

Reads exactly len bytes from this file into the byte array, starting at the current file pointer. This method reads repeatedly from the file until the requested number of bytes are read. This method blocks until the requested number of bytes are read, the end of the stream is detected, or an exception is thrown.

Specified by:

readFully in interface DataInput

Parameters:

b - the buffer into which the data is read.

off - the start offset of the data.

len - the number of bytes to read.

Throws:

EOFException - if this file reaches the end before reading all the bytes.

IOException - if an I/O error occurs.

skipBytes

public int skipBytes(int n)
              throws IOException   

Attempts to skip over n bytes of input discarding the skipped bytes.

This method may skip over some smaller number of bytes, possibly zero. This may result from any of a number of conditions; reaching end of file before n bytes have been skipped is only one possibility. This method never throws an EOFException. The actual number of bytes skipped is returned. If n is negative, no bytes are skipped.

Specified by:

skipBytes in interface DataInput

Parameters:

n - the number of bytes to be skipped.

Returns:

the actual number of bytes skipped.

Throws:

IOException - if an I/O error occurs.

write

public void write(int b)
           throws IOException   

Writes the specified byte to this file. The write starts at the current file pointer.

Specified by:

write in interface DataOutput

Parameters:

b - the byte to be written.

Throws:

IOException - if an I/O error occurs.

write

public void write(byte[] b)
           throws IOException   

Writes b.length bytes from the specified byte array to this file, starting at the current file pointer.

Specified by:

write in interface DataOutput

Parameters:

b - the data.

Throws:

IOException - if an I/O error occurs.

write

public void write(byte[] b,
                  int off,
                  int len)
           throws IOException   

Writes len bytes from the specified byte array starting at offset off to this file.

Specified by:

write in interface DataOutput

Parameters:

b - the data.

off - the start offset in the data.

len - the number of bytes to write.

Throws:

IOException - if an I/O error occurs.

getFilePointer

public long getFilePointer()
                    throws IOException   

Returns the current offset in this file.

Returns:

the offset from the beginning of the file, in bytes, at which the next read or write occurs.

Throws:

IOException - if an I/O error occurs.

seek

public void seek(long pos)
          throws