Java Doc for Console.java in  » 6.0-JDK-Core » io-nio » java » io » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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 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 Console.readLine() , Console.readPassword() , Console.format format() , Console.printf printf() as well as the read, format and write operations on the objects returned by Console.reader() and Console.writer() may block in multithreaded scenarios.

Invoking close() on the objects returned by the Console.reader() and the Console.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 NullPointerException to be thrown.

Security note: If an application needs to read a password or other secure data, it should use Console.readPassword() or Console.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, ' ');
 }
 

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)

This method is intended to be used by sophisticated applications, for example, a 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 Console.readLine .

The bulk read operations java.io.Reader.read(char[]) read(char[]) , java.io.Reader.read(char[]intint) read(char[], int, int) and 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.

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)

This method is intended to be used by sophisticated applications, for example, a 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 Console.readLine .

The bulk read operations java.io.Reader.read(char[]) read(char[]) , java.io.Reader.read(char[]intint) read(char[], int, int) and 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. The reader associated with this console