Class System

The standard output PrintStream. This is assigned at startup and starts its life perfectly valid. Although it is marked final, you can change it using {@link #setErr(PrintStream)} through some hefty VM magic.

This corresponds to the C stderr and C++ cerr variables, which typically output error messages to the screen, but may be used to pipe output to other processes or files. That should all be transparent to you, however.

The standard InputStream. This is assigned at startup and starts its life perfectly valid. Although it is marked final, you can change it using {@link #setIn(InputStream)} through some hefty VM magic.

This corresponds to the C stdin and C++ cin variables, which typically input from the keyboard, but may be used to pipe input from other processes or files. That should all be transparent to you, however.

The standard output PrintStream. This is assigned at startup and starts its life perfectly valid. Although it is marked final, you can change it using {@link #setOut(PrintStream)} through some hefty VM magic.

This corresponds to the C stdout and C++ cout variables, which typically output normal messages to the screen, but may be used to pipe output to other processes or files. That should all be transparent to you, however.

Copy one array onto another from src[srcStart] ... src[srcStart+len-1] to dest[destStart] ... dest[destStart+len-1]. First, the arguments are validated: neither array may be null, they must be of compatible types, and the start and length must fit within both arrays. Then the copying starts, and proceeds through increasing slots. If src and dest are the same array, this will appear to copy the data to a temporary location first. An ArrayStoreException in the middle of copying will leave earlier elements copied, but later elements unchanged.

Parameters: src the array to copy elements from srcStart the starting position in src dest the array to copy elements to destStart the starting position in dest len the number of elements to copy

Throws: NullPointerException if src or dest is null ArrayStoreException if src or dest is not an array, if they are not compatible array types, or if an incompatible runtime type is stored in dest IndexOutOfBoundsException if len is negative, or if the start or end copy position in either array is out of bounds

Remove a single system property by name. A security check may be performed, checkPropertyAccess(key, "write").

Parameters: key the name of the system property to remove

Returns: the previous value, or null

Throws: SecurityException if permission is denied NullPointerException if key is null IllegalArgumentException if key is ""

Since: 1.5

Get the current time, measured in the number of milliseconds from the beginning of Jan. 1, 1970. This is gathered from the system clock, with any attendant incorrectness (it may be timezone dependent).

Returns: the current time

See Also: Date

Terminate the Virtual Machine. This just calls Runtime.getRuntime().exit(status), and never returns. Obviously, a security check is in order, checkExit.

Parameters: status the exit status; by convention non-zero is abnormal

Throws: SecurityException if permission is denied

See Also: Runtime

Calls the garbage collector. This is only a hint, and it is up to the implementation what this hint suggests, but it usually causes a best-effort attempt to reclaim unused memory from discarded objects. This calls Runtime.getRuntime().gc().

See Also: gc

Gets the value of an environment variable.

Parameters: name the name of the environment variable

Returns: the string value of the variable or null when the environment variable is not defined.

Throws: NullPointerException SecurityException if permission is denied

Since: 1.5

UNKNOWN: This method was deprecated in some JDK releases, but was restored in 1.5.

Returns an unmodifiable view of the system environment variables. If the underlying system does not support environment variables, an empty map is returned.

The returned map is read-only and does not accept queries using null keys or values, or those of a type other than String. Attempts to modify the map will throw an UnsupportedOperationException, while attempts to pass in a null value will throw a NullPointerException. Types other than String throw a ClassCastException.

As the returned map is generated using data from the underlying platform, it may not comply with the equals() and hashCode() contracts. It is also likely that the keys of this map will be case-sensitive.

Use of this method may require a security check for the RuntimePermission "getenv.*".

Returns: a map of the system environment variables.

Throws: SecurityException if the checkPermission method of an installed security manager prevents access to the system environment variables.

Since: 1.5

Get all the system properties at once. A security check may be performed, checkPropertiesAccess. Note that a security manager may allow getting a single property, but not the entire group.

The required properties include:

java.version

Java version number

java.vendor

Java vendor specific string

java.vendor.url

Java vendor URL

java.home

Java installation directory

java.vm.specification.version

VM Spec version

java.vm.specification.vendor

VM Spec vendor

java.vm.specification.name

VM Spec name

java.vm.version

VM implementation version

java.vm.vendor

VM implementation vendor

java.vm.name

VM implementation name

java.specification.version

Java Runtime Environment version

java.specification.vendor

Java Runtime Environment vendor

java.specification.name

Java Runtime Environment name

java.class.version

Java class version number

java.class.path

Java classpath

java.library.path

Path for finding Java libraries

java.io.tmpdir

Default temp file path

java.compiler

Name of JIT to use

java.ext.dirs

Java extension path

os.name

Operating System Name

os.arch

Operating System Architecture

os.version

Operating System Version

file.separator

File separator ("/" on Unix)

path.separator

Path separator (":" on Unix)

line.separator

Line separator ("\n" on Unix)

user.name

User account name

user.home

User home directory

user.dir

User's current working directory

In addition, gnu defines several other properties, where ? stands for each character in '0' through '9':

gnu.classpath.home

Path to the classpath libraries.

gnu.classpath.version

Version of the classpath libraries.

gnu.classpath.vm.shortname

Succinct version of the VM name; used for finding property files in file system

gnu.classpath.home.url

Base URL; used for finding property files in file system

gnu.cpu.endian

big or little

gnu.java.io.encoding_scheme_alias.iso-8859-?

8859_?

gnu.java.io.encoding_scheme_alias.iso8859_?

8859_?

gnu.java.io.encoding_scheme_alias.iso-latin-_?

8859_?

gnu.java.io.encoding_scheme_alias.latin?

8859_?

gnu.java.io.encoding_scheme_alias.utf-8

UTF8

gnu.java.util.zoneinfo.dir

Root of zoneinfo tree

gnu.javax.print.server

Hostname of external CUPS server.

Returns: the system properties, will never be null

Throws: SecurityException if permission is denied

Get a single system property by name. A security check may be performed, checkPropertyAccess(key).

Parameters: key the name of the system property to get

Returns: the property, or null if not found

Throws: SecurityException if permission is denied NullPointerException if key is null IllegalArgumentException if key is ""

Get a single system property by name. A security check may be performed, checkPropertyAccess(key).

Parameters: key the name of the system property to get def the default

Returns: the property, or def if not found

Throws: SecurityException if permission is denied NullPointerException if key is null IllegalArgumentException if key is ""

Get the current SecurityManager. If the SecurityManager has not been set yet, then this method returns null.

Returns: the current SecurityManager, or null

Get a hash code computed by the VM for the Object. This hash code will be the same as Object's hashCode() method. It is usually some convolution of the pointer to the Object internal to the VM. It follows standard hash code rules, in that it will remain the same for a given Object for the lifetime of that Object.

Parameters: o the Object to get the hash code for

Returns: the VM-dependent hash code for this Object

Since: 1.1

Load a code file using its explicit system-dependent filename. A security check may be performed, checkLink. This just calls Runtime.getRuntime().load(filename).

The library is loaded using the class loader associated with the class associated with the invoking method.

Parameters: filename the code file to load

Throws: SecurityException if permission is denied UnsatisfiedLinkError if the file cannot be loaded

See Also: load

Load a library using its explicit system-dependent filename. A security check may be performed, checkLink. This just calls Runtime.getRuntime().load(filename).

The library is loaded using the class loader associated with the class associated with the invoking method.

Parameters: libname the library file to load

Throws: SecurityException if permission is denied UnsatisfiedLinkError if the file cannot be loaded

See Also: load

Convert a library name to its platform-specific variant.

Parameters: libname the library name, as used in loadLibrary

Returns: the platform-specific mangling of the name

Since: 1.2

Returns the current value of a nanosecond-precise system timer. The value of the timer is an offset relative to some arbitrary fixed time, which may be in the future (making the value negative). This method is useful for timing events where nanosecond precision is required. This is achieved by calling this method before and after the event, and taking the difference betweent the two times:

long startTime = System.nanoTime();
... event code ...
long endTime = System.nanoTime();
long duration = endTime - startTime;

Note that the value is only nanosecond-precise, and not accurate; there is no guarantee that the difference between two values is really a nanosecond. Also, the value is prone to overflow if the offset exceeds 2^63.

Returns: the time of a system timer in nanoseconds.

Since: 1.5

Runs object finalization on pending objects. This is only a hint, and it is up to the implementation what this hint suggests, but it usually causes a best-effort attempt to run finalizers on all objects ready to be reclaimed. This calls Runtime.getRuntime().runFinalization().

See Also: runFinalization

Deprecated: never rely on finalizers to do a clean, thread-safe, mop-up from your code

Tell the Runtime whether to run finalization before exiting the JVM. This is inherently unsafe in multi-threaded applications, since it can force initialization on objects which are still in use by live threads, leading to deadlock; therefore this is disabled by default. There may be a security check, checkExit(0). This calls Runtime.runFinalizersOnExit().

Parameters: finalizeOnExit whether to run finalizers on exit

Throws: SecurityException if permission is denied

Since: 1.1

See Also: Runtime

Set {@link #err} to a new PrintStream. This uses some VM magic to change a "final" variable, so naturally there is a security check, RuntimePermission("setIO").

Parameters: err the new PrintStream

Throws: SecurityException if permission is denied

Since: 1.1

Set {@link #in} to a new InputStream. This uses some VM magic to change a "final" variable, so naturally there is a security check, RuntimePermission("setIO").

Parameters: in the new InputStream

Throws: SecurityException if permission is denied

Since: 1.1

Set {@link #out} to a new PrintStream. This uses some VM magic to change a "final" variable, so naturally there is a security check, RuntimePermission("setIO").

Parameters: out the new PrintStream

Throws: SecurityException if permission is denied

Since: 1.1

Set all the system properties at once. A security check may be performed, checkPropertiesAccess. Note that a security manager may allow setting a single property, but not the entire group. An argument of null resets the properties to the startup default.

Parameters: properties the new set of system properties

Throws: SecurityException if permission is denied

Set a single system property by name. A security check may be performed, checkPropertyAccess(key, "write").

Parameters: key the name of the system property to set value the new value

Returns: the previous value, or null

Throws: SecurityException if permission is denied NullPointerException if key is null IllegalArgumentException if key is ""

Since: 1.2

Set the current SecurityManager. If a security manager already exists, then RuntimePermission("setSecurityManager") is checked first. Since this permission is denied by the default security manager, setting the security manager is often an irreversible action. Spec Note: Don't ask me, I didn't write it. It looks pretty vulnerable; whoever gets to the gate first gets to set the policy. There is probably some way to set the original security manager as a command line argument to the VM, but I don't know it.

Parameters: sm the new SecurityManager

Throws: SecurityException if permission is denied