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Console.javaAPI DocJava SE 6 API20599Tue Jun 10 00:25:32 BST 2008java.io

Console

public final class Console extends Object implements Flushable
Methods to access the character-based console device, if any, associated with the current Java virtual machine.

Whether a virtual machine has a console is dependent upon the underlying platform and also upon the manner in which the virtual machine is invoked. If the virtual machine is started from an interactive command line without redirecting the standard input and output streams then its console will exist and will typically be connected to the keyboard and display from which the virtual machine was launched. If the virtual machine is started automatically, for example by a background job scheduler, then it will typically not have a console.

If this virtual machine has a console then it is represented by a unique instance of this class which can be obtained by invoking the {@link java.lang.System#console()} method. If no console device is available then an invocation of that method will return null.

Read and write operations are synchronized to guarantee the atomic completion of critical operations; therefore invoking methods {@link #readLine()}, {@link #readPassword()}, {@link #format format()}, {@link #printf printf()} as well as the read, format and write operations on the objects returned by {@link #reader()} and {@link #writer()} may block in multithreaded scenarios.

Invoking close() on the objects returned by the {@link #reader()} and the {@link #writer()} will not close the underlying stream of those objects.

The console-read methods return null when the end of the console input stream is reached, for example by typing control-D on Unix or control-Z on Windows. Subsequent read operations will succeed if additional characters are later entered on the console's input device.

Unless otherwise specified, passing a null argument to any method in this class will cause a {@link NullPointerException} to be thrown.

Security note: If an application needs to read a password or other secure data, it should use {@link #readPassword()} or {@link #readPassword(String, Object...)} and manually zero the returned character array after processing to minimize the lifetime of sensitive data in memory.

Console cons;
char[] passwd;
if ((cons = System.console()) != null &&
(passwd = cons.readPassword("[%s]", "Password:")) != null) {
...
java.util.Arrays.fill(passwd, ' ');
}
author
Xueming Shen
version
1.11, 06/12/06
since
1.6

Fields Summary
private Object
readLock
private Object
writeLock
private Reader
reader
private Writer
out
private PrintWriter
pw
private Formatter
formatter
private Charset
cs
private char[]
rcb
private static boolean
echoOff
private static Console
cons
Constructors Summary
private Console()

        readLock = new Object();
        writeLock = new Object();    
        String csname = encoding();
        if (csname != null) {
            try {
                cs = Charset.forName(csname);
            } catch (Exception x) {}
        }
        if (cs == null)
            cs = Charset.defaultCharset();
        out = StreamEncoder.forOutputStreamWriter(
                  new FileOutputStream(FileDescriptor.out),
		  writeLock, 
		  cs);
        pw = new PrintWriter(out, true) { public void close() {} };
        formatter = new Formatter(out);
        reader = new LineReader(StreamDecoder.forInputStreamReader(
		     new FileInputStream(FileDescriptor.in),
		     readLock, 
		     cs));
	rcb = new char[1024];
    
Methods Summary
private static native booleanecho(boolean on)

private static native java.lang.Stringencoding()

public voidflush()
Flushes the console and forces any buffered output to be written immediately .

        pw.flush();
    
public java.io.Consoleformat(java.lang.String fmt, java.lang.Object args)
Writes a formatted string to this console's output stream using the specified format string and arguments.

param
fmt A format string as described in Format string syntax
param
args Arguments referenced by the format specifiers in the format string. If there are more arguments than format specifiers, the extra arguments are ignored. The number of arguments is variable and may be zero. The maximum number of arguments is limited by the maximum dimension of a Java array as defined by the Java Virtual Machine Specification. The behaviour on a null argument depends on the conversion.
throws
IllegalFormatException If a format string contains an illegal syntax, a format specifier that is incompatible with the given arguments, insufficient arguments given the format string, or other illegal conditions. For specification of all possible formatting errors, see the Details section of the formatter class specification.
return
This console

        formatter.format(fmt, args).flush();
        return this;
    
private char[]grow()

        assert Thread.holdsLock(readLock);
        char[] t = new char[rcb.length * 2];
        System.arraycopy(rcb, 0, t, 0, rcb.length);
        rcb = t;
        return rcb;
    
private static native booleanistty()

public java.io.Consoleprintf(java.lang.String format, java.lang.Object args)
A convenience method to write a formatted string to this console's output stream using the specified format string and arguments.

An invocation of this method of the form con.printf(format, args) behaves in exactly the same way as the invocation of

con.format(format, args)
.

param
format A format string as described in Format string syntax.
param
args Arguments referenced by the format specifiers in the format string. If there are more arguments than format specifiers, the extra arguments are ignored. The number of arguments is variable and may be zero. The maximum number of arguments is limited by the maximum dimension of a Java array as defined by the Java Virtual Machine Specification. The behaviour on a null argument depends on the conversion.
throws
IllegalFormatException If a format string contains an illegal syntax, a format specifier that is incompatible with the given arguments, insufficient arguments given the format string, or other illegal conditions. For specification of all possible formatting errors, see the Details section of the formatter class specification.
return
This console

	return format(format, args);
    
public java.lang.StringreadLine(java.lang.String fmt, java.lang.Object args)
Provides a formatted prompt, then reads a single line of text from the console.

param
fmt A format string as described in Format string syntax.
param
args Arguments referenced by the format specifiers in the format string. If there are more arguments than format specifiers, the extra arguments are ignored. The maximum number of arguments is limited by the maximum dimension of a Java array as defined by the Java Virtual Machine Specification.
throws
IllegalFormatException If a format string contains an illegal syntax, a format specifier that is incompatible with the given arguments, insufficient arguments given the format string, or other illegal conditions. For specification of all possible formatting errors, see the Details section of the formatter class specification.
throws
IOError If an I/O error occurs.
return
A string containing the line read from the console, not including any line-termination characters, or null if an end of stream has been reached.

        String line = null;
        synchronized (writeLock) {  
            synchronized(readLock) {
                if (fmt.length() != 0)
                    pw.format(fmt, args);
                try {
                    char[] ca = readline(false);
                    if (ca != null)
                        line = new String(ca);
                } catch (IOException x) {
                    throw new IOError(x);
                }
            }
        }
        return line;
    
public java.lang.StringreadLine()
Reads a single line of text from the console.

throws
IOError If an I/O error occurs.
return
A string containing the line read from the console, not including any line-termination characters, or null if an end of stream has been reached.

        return readLine("");
    
public char[]readPassword(java.lang.String fmt, java.lang.Object args)
Provides a formatted prompt, then reads a password or passphrase from the console with echoing disabled.

param
fmt A format string as described in Format string syntax for the prompt text.
param
args Arguments referenced by the format specifiers in the format string. If there are more arguments than format specifiers, the extra arguments are ignored. The maximum number of arguments is limited by the maximum dimension of a Java array as defined by the Java Virtual Machine Specification.
throws
IllegalFormatException If a format string contains an illegal syntax, a format specifier that is incompatible with the given arguments, insufficient arguments given the format string, or other illegal conditions. For specification of all possible formatting errors, see the Details section of the formatter class specification.
throws
IOError If an I/O error occurs.
return
A character array containing the password or passphrase read from the console, not including any line-termination characters, or null if an end of stream has been reached.

        char[] passwd = null;
        synchronized (writeLock) {  
            synchronized(readLock) {
                if (fmt.length() != 0)
                    pw.format(fmt, args);
                try {
                    echoOff = echo(false);
                    passwd = readline(true);
                } catch (IOException x) {
                    throw new IOError(x);
                } finally {
		    try {
                        echoOff = echo(true);
                    } catch (IOException xx) {}
                }
                pw.println();
	    }
	}
        return passwd;
    
public char[]readPassword()
Reads a password or passphrase from the console with echoing disabled

throws
IOError If an I/O error occurs.
return
A character array containing the password or passphrase read from the console, not including any line-termination characters, or null if an end of stream has been reached.

        return readPassword("");
    
public java.io.Readerreader()
Retrieves the unique {@link java.io.Reader Reader} object associated with this console.

This method is intended to be used by sophisticated applications, for example, a {@link java.util.Scanner} object which utilizes the rich parsing/scanning functionality provided by the Scanner:

Console con = System.console();
if (con != null) {
Scanner sc = new Scanner(con.reader());
...
}

For simple applications requiring only line-oriented reading, use {@link #readLine}.

The bulk read operations {@link java.io.Reader#read(char[]) read(char[]) }, {@link java.io.Reader#read(char[], int, int) read(char[], int, int) } and {@link java.io.Reader#read(java.nio.CharBuffer) read(java.nio.CharBuffer)} on the returned object will not read in characters beyond the line bound for each invocation, even if the destination buffer has space for more characters. A line bound is considered to be any one of a line feed ('\n'), a carriage return ('\r'), a carriage return followed immediately by a linefeed, or an end of stream.

return
The reader associated with this console

        return reader;
    
private char[]readline(boolean zeroOut)

        int len = reader.read(rcb, 0, rcb.length);
        if (len < 0) 
            return null;  //EOL
        if (rcb[len-1] == '\r")
            len--;        //remove CR at end;
	else if (rcb[len-1] == '\n") {
            len--;        //remove LF at end;
	    if (len > 0 && rcb[len-1] == '\r")
                len--;    //remove the CR, if there is one
        }
	char[] b = new char[len];
        if (len > 0) {
	    System.arraycopy(rcb, 0, b, 0, len);
            if (zeroOut) {
                Arrays.fill(rcb, 0, len, ' ");
            }
	}
	return b;
    
public java.io.PrintWriterwriter()
Retrieves the unique {@link java.io.PrintWriter PrintWriter} object associated with this console.

return
The printwriter associated with this console

        return pw;