Class System

public static final PrintStream err

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 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.

public static final InputStream in

The standard InputStream. This is assigned at startup and starts its life perfectly valid. Although it is marked final, you can change it using 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.

public static final PrintStream out

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 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.

public static MapString> getenv()

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

public static void arraycopy(Object src,
                             int srcStart,
                             Object dest,
                             int destStart,
                             int len)

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

public static String clearProperty(String key)

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

public static long currentTimeMillis()

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

public static void exit(int status)

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.exit(int)

public static void gc()

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:

Runtime.gc()

public static Properties getProperties()

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:

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

Returns:

the system properties, will never be null

Throws:

SecurityException - if permission is denied

public static String getProperty(String key)

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 ""

public static String getProperty(String key,
                                 String def)

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 ""

public static SecurityManager getSecurityManager()

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

Returns:

the current SecurityManager, or null

public static String getenv(String name)

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

public static int identityHashCode(Object o)

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

public static Channel inheritedChannel()
            throws IOException

Returns the inherited channel of the VM. This wraps the inheritedChannel() call of the system's default SelectorProvider.

Returns:

the inherited channel of the VM

Throws:

IOException - If an I/O error occurs

SecurityException - If an installed security manager denies access to RuntimePermission("inheritedChannel")

Since:

1.5

public static void load(String filename)

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

See Also:

Runtime.load(String)

public static void loadLibrary(String libname)

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

See Also:

Runtime.load(String)

public static String mapLibraryName(String libname)

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

public static long nanoTime()

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

public static void runFinalization()

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:

Runtime.runFinalization()

public static void runFinalizersOnExit(boolean finalizeOnExit)

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.runFinalizersOnExit(boolean)

public static void setErr(PrintStream err)

Set 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

public static void setIn(InputStream in)

Set 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

public static void setOut(PrintStream out)

Set 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

public static void setProperties(Properties properties)

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

public static String setProperty(String key,
                                 String value)

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

public static void setSecurityManager(SecurityManager sm)

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