Runtime
public
class
Runtime
extends Object
java.lang.Object | |
↳ | java.lang.Runtime |
Every Java application has a single instance of class
Runtime
that allows the application to interface with
the environment in which the application is running. The current
runtime can be obtained from the getRuntime
method.
An application cannot create its own instance of this class.
See also:
Summary
Public methods | |
---|---|
void
|
addShutdownHook(Thread hook)
Registers a new virtual-machine shutdown hook. |
int
|
availableProcessors()
Returns the number of processors available to the Java virtual machine. |
Process
|
exec(String[] cmdarray)
Executes the specified command and arguments in a separate process. |
Process
|
exec(String[] cmdarray, String[] envp)
Executes the specified command and arguments in a separate process with the specified environment. |
Process
|
exec(String command)
Executes the specified string command in a separate process. |
Process
|
exec(String command, String[] envp)
Executes the specified string command in a separate process with the specified environment. |
Process
|
exec(String[] cmdarray, String[] envp, File dir)
Executes the specified command and arguments in a separate process with the specified environment and working directory. |
Process
|
exec(String command, String[] envp, File dir)
Executes the specified string command in a separate process with the specified environment and working directory. |
void
|
exit(int status)
Terminates the currently running Java virtual machine by initiating its shutdown sequence. |
long
|
freeMemory()
Returns the amount of free memory in the Java Virtual Machine. |
void
|
gc()
Runs the garbage collector. |
static
Runtime
|
getRuntime()
Returns the runtime object associated with the current Java application. |
void
|
halt(int status)
Forcibly terminates the currently running Java virtual machine. |
void
|
load(String filename)
Loads the native library specified by the filename argument. |
void
|
loadLibrary(String libname)
Loads the native library specified by the |
long
|
maxMemory()
Returns the maximum amount of memory that the Java virtual machine will attempt to use. |
boolean
|
removeShutdownHook(Thread hook)
De-registers a previously-registered virtual-machine shutdown hook. |
void
|
runFinalization()
Runs the finalization methods of any objects pending finalization. |
static
void
|
runFinalizersOnExit(boolean value)
This method was deprecated in API level 15. This method is inherently unsafe. It may result in finalizers being called on live objects while other threads are concurrently manipulating those objects, resulting in erratic behavior or deadlock. |
long
|
totalMemory()
Returns the total amount of memory in the Java virtual machine. |
void
|
traceInstructions(boolean on)
This method was deprecated in API level 34. This method was intended to control instruction tracing. It has been superseded by ART-specific tracing mechanisms. |
void
|
traceMethodCalls(boolean on)
This method was deprecated in API level 34. This method was intended to control method call tracing. It has been superseded by ART-specific tracing mechanisms. |
Inherited methods | |
---|---|
Public methods
addShutdownHook
public void addShutdownHook (Thread hook)
Registers a new virtual-machine shutdown hook.
The Java virtual machine shuts down in response to two kinds of events:
- The program exits normally, when the last non-daemon
thread exits or when the
exit
(equivalently,System.exit
) method is invoked, or - The virtual machine is terminated in response to a
user interrupt, such as typing
^C
, or a system-wide event, such as user logoff or system shutdown.
A shutdown hook is simply an initialized but unstarted
thread. When the virtual machine begins its shutdown sequence it will
start all registered shutdown hooks in some unspecified order and let
them run concurrently. When all the hooks have finished it will then
halt. Note that daemon threads will continue to run during the shutdown
sequence, as will non-daemon threads if shutdown was initiated by
invoking the exit
method.
Once the shutdown sequence has begun it can be stopped only by
invoking the halt
method, which forcibly
terminates the virtual machine.
Once the shutdown sequence has begun it is impossible to register a
new shutdown hook or de-register a previously-registered hook.
Attempting either of these operations will cause an
IllegalStateException
to be thrown.
Shutdown hooks run at a delicate time in the life cycle of a virtual machine and should therefore be coded defensively. They should, in particular, be written to be thread-safe and to avoid deadlocks insofar as possible. They should also not rely blindly upon services that may have registered their own shutdown hooks and therefore may themselves in the process of shutting down. Attempts to use other thread-based services such as the AWT event-dispatch thread, for example, may lead to deadlocks.
Shutdown hooks should also finish their work quickly. When a
program invokes exit
the expectation is
that the virtual machine will promptly shut down and exit. When the
virtual machine is terminated due to user logoff or system shutdown the
underlying operating system may only allow a fixed amount of time in
which to shut down and exit. It is therefore inadvisable to attempt any
user interaction or to perform a long-running computation in a shutdown
hook.
Uncaught exceptions are handled in shutdown hooks just as in any
other thread, by invoking the
uncaughtException
method of the
thread's ThreadGroup
object. The default implementation of this
method prints the exception's stack trace to System#err
and
terminates the thread; it does not cause the virtual machine to exit or
halt.
In rare circumstances the virtual machine may abort, that is,
stop running without shutting down cleanly. This occurs when the
virtual machine is terminated externally, for example with the
SIGKILL
signal on Unix or the TerminateProcess
call on
Microsoft Windows. The virtual machine may also abort if a native
method goes awry by, for example, corrupting internal data structures or
attempting to access nonexistent memory. If the virtual machine aborts
then no guarantee can be made about whether or not any shutdown hooks
will be run.
Parameters | |
---|---|
hook |
Thread : An initialized but unstarted Thread object |
Throws | |
---|---|
IllegalArgumentException |
If the specified hook has already been registered, or if it can be determined that the hook is already running or has already been run |
IllegalStateException |
If the virtual machine is already in the process of shutting down |
SecurityException |
If a security manager is present and it denies
RuntimePermission ("shutdownHooks") |
availableProcessors
public int availableProcessors ()
Returns the number of processors available to the Java virtual machine.
This value may change during a particular invocation of the virtual machine. Applications that are sensitive to the number of available processors should therefore occasionally poll this property and adjust their resource usage appropriately.
Returns | |
---|---|
int |
the maximum number of processors available to the virtual machine; never smaller than one |
exec
public Process exec (String[] cmdarray)
Executes the specified command and arguments in a separate process.
This is a convenience method. An invocation of the form
exec(cmdarray)
behaves in exactly the same way as the invocation
exec
(cmdarray, null, null)
.
Parameters | |
---|---|
cmdarray |
String : array containing the command to call and
its arguments. |
Returns | |
---|---|
Process |
A new Process object for managing the subprocess |
Throws | |
---|---|
SecurityException |
If a security manager exists and its
checkExec
method doesn't allow creation of the subprocess |
IOException |
If an I/O error occurs |
NullPointerException |
If cmdarray is null ,
or one of the elements of cmdarray is null |
IndexOutOfBoundsException |
If cmdarray is an empty array
(has length 0 ) |
See also:
exec
public Process exec (String[] cmdarray, String[] envp)
Executes the specified command and arguments in a separate process with the specified environment.
This is a convenience method. An invocation of the form
exec(cmdarray, envp)
behaves in exactly the same way as the invocation
exec
(cmdarray, envp, null)
.
Parameters | |
---|---|
cmdarray |
String : array containing the command to call and
its arguments. |
envp |
String : array of strings, each element of which
has environment variable settings in the format
name=value, or
null if the subprocess should inherit
the environment of the current process. |
Returns | |
---|---|
Process |
A new Process object for managing the subprocess |
Throws | |
---|---|
SecurityException |
If a security manager exists and its
checkExec
method doesn't allow creation of the subprocess |
IOException |
If an I/O error occurs |
NullPointerException |
If cmdarray is null ,
or one of the elements of cmdarray is null ,
or one of the elements of envp is null |
IndexOutOfBoundsException |
If cmdarray is an empty array
(has length 0 ) |
See also:
exec
public Process exec (String command)
Executes the specified string command in a separate process.
This is a convenience method. An invocation of the form
exec(command)
behaves in exactly the same way as the invocation
exec
(command, null, null)
.
Parameters | |
---|---|
command |
String : a specified system command. |
Returns | |
---|---|
Process |
A new Process object for managing the subprocess |
Throws | |
---|---|
SecurityException |
If a security manager exists and its
checkExec
method doesn't allow creation of the subprocess |
IOException |
If an I/O error occurs |
NullPointerException |
If command is null |
IllegalArgumentException |
If command is empty |
exec
public Process exec (String command, String[] envp)
Executes the specified string command in a separate process with the specified environment.
This is a convenience method. An invocation of the form
exec(command, envp)
behaves in exactly the same way as the invocation
exec
(command, envp, null)
.
Parameters | |
---|---|
command |
String : a specified system command. |
envp |
String : array of strings, each element of which
has environment variable settings in the format
name=value, or
null if the subprocess should inherit
the environment of the current process. |
Returns | |
---|---|
Process |
A new Process object for managing the subprocess |
Throws | |
---|---|
SecurityException |
If a security manager exists and its
checkExec
method doesn't allow creation of the subprocess |
IOException |
If an I/O error occurs |
NullPointerException |
If command is null ,
or one of the elements of envp is null |
IllegalArgumentException |
If command is empty |
exec
public Process exec (String[] cmdarray, String[] envp, File dir)
Executes the specified command and arguments in a separate process with the specified environment and working directory.
Given an array of strings cmdarray
, representing the
tokens of a command line, and an array of strings envp
,
representing "environment" variable settings, this method creates
a new process in which to execute the specified command.
This method checks that cmdarray
is a valid operating
system command. Which commands are valid is system-dependent,
but at the very least the command must be a non-empty list of
non-null strings.
If envp
is null
, the subprocess inherits the
environment settings of the current process.
A minimal set of system dependent environment variables may be required to start a process on some operating systems. As a result, the subprocess may inherit additional environment variable settings beyond those in the specified environment.
ProcessBuilder#start()
is now the preferred way to
start a process with a modified environment.
The working directory of the new subprocess is specified by dir
.
If dir
is null
, the subprocess inherits the
current working directory of the current process.
If a security manager exists, its
checkExec
method is invoked with the first component of the array
cmdarray
as its argument. This may result in a
SecurityException
being thrown.
Starting an operating system process is highly system-dependent. Among the many things that can go wrong are:
- The operating system program file was not found.
- Access to the program file was denied.
- The working directory does not exist.
In such cases an exception will be thrown. The exact nature
of the exception is system-dependent, but it will always be a
subclass of IOException
.
If the operating system does not support the creation of
processes, an UnsupportedOperationException
will be thrown.
Parameters | |
---|---|
cmdarray |
String : array containing the command to call and
its arguments. |
envp |
String : array of strings, each element of which
has environment variable settings in the format
name=value, or
null if the subprocess should inherit
the environment of the current process. |
dir |
File : the working directory of the subprocess, or
null if the subprocess should inherit
the working directory of the current process. |
Returns | |
---|---|
Process |
A new Process object for managing the subprocess |
Throws | |
---|---|
SecurityException |
If a security manager exists and its
checkExec
method doesn't allow creation of the subprocess |
UnsupportedOperationException |
If the operating system does not support the creation of processes. |
IOException |
If an I/O error occurs |
NullPointerException |
If cmdarray is null ,
or one of the elements of cmdarray is null ,
or one of the elements of envp is null |
IndexOutOfBoundsException |
If cmdarray is an empty array
(has length 0 ) |
See also:
exec
public Process exec (String command, String[] envp, File dir)
Executes the specified string command in a separate process with the specified environment and working directory.
This is a convenience method. An invocation of the form
exec(command, envp, dir)
behaves in exactly the same way as the invocation
exec
(cmdarray, envp, dir)
,
where cmdarray
is an array of all the tokens in
command
.
More precisely, the command
string is broken
into tokens using a StringTokenizer
created by the call
new {@link StringTokenizer}(command)
with no
further modification of the character categories. The tokens
produced by the tokenizer are then placed in the new string
array cmdarray
, in the same order.
Parameters | |
---|---|
command |
String : a specified system command. |
envp |
String : array of strings, each element of which
has environment variable settings in the format
name=value, or
null if the subprocess should inherit
the environment of the current process. |
dir |
File : the working directory of the subprocess, or
null if the subprocess should inherit
the working directory of the current process. |
Returns | |
---|---|
Process |
A new Process object for managing the subprocess |
Throws | |
---|---|
SecurityException |
If a security manager exists and its
checkExec
method doesn't allow creation of the subprocess |
IOException |
If an I/O error occurs |
NullPointerException |
If command is null ,
or one of the elements of envp is null |
IllegalArgumentException |
If command is empty |
See also:
exit
public void exit (int status)
Terminates the currently running Java virtual machine by initiating its shutdown sequence. This method never returns normally. The argument serves as a status code; by convention, a nonzero status code indicates abnormal termination.
All registered shutdown hooks, if any, are started in some unspecified order and allowed to run concurrently until they finish. Once this is done the virtual machine halts.
If this method is invoked after all shutdown hooks have already been run and the status is nonzero then this method halts the virtual machine with the given status code. Otherwise, this method blocks indefinitely.
The System.exit
method is the
conventional and convenient means of invoking this method.
Parameters | |
---|---|
status |
int : Termination status. By convention, a nonzero status code
indicates abnormal termination. |
Throws | |
---|---|
SecurityException |
If a security manager is present and its
checkExit method does not permit
exiting with the specified status |
freeMemory
public long freeMemory ()
Returns the amount of free memory in the Java Virtual Machine.
Calling the
gc
method may result in increasing the value returned
by freeMemory.
Returns | |
---|---|
long |
an approximation to the total amount of memory currently available for future allocated objects, measured in bytes. |
gc
public void gc ()
Runs the garbage collector. Calling this method suggests that the Java virtual machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse. When control returns from the method call, the virtual machine has made its best effort to recycle all discarded objects.
The name gc
stands for "garbage
collector". The virtual machine performs this recycling
process automatically as needed, in a separate thread, even if the
gc
method is not invoked explicitly.
The method System#gc()
is the conventional and convenient
means of invoking this method.
getRuntime
public static Runtime getRuntime ()
Returns the runtime object associated with the current Java application.
Most of the methods of class Runtime
are instance
methods and must be invoked with respect to the current runtime object.
Returns | |
---|---|
Runtime |
the Runtime object associated with the current
Java application. |
halt
public void halt (int status)
Forcibly terminates the currently running Java virtual machine. This method never returns normally.
This method should be used with extreme caution. Unlike the
exit
method, this method does not cause shutdown
hooks to be started. If the shutdown sequence has already been
initiated then this method does not wait for any running
shutdown hooks to finish their work.
Parameters | |
---|---|
status |
int : Termination status. By convention, a nonzero status code
indicates abnormal termination. If the exit
(equivalently, System.exit ) method
has already been invoked then this status code
will override the status code passed to that method. |
Throws | |
---|---|
SecurityException |
If a security manager is present and its
checkExit method
does not permit an exit with the specified status |
load
public void load (String filename)
Loads the native library specified by the filename argument. The filename
argument must be an absolute path name.
(for example
Runtime.getRuntime().load("/home/avh/lib/libX11.so");
).
If the filename argument, when stripped of any platform-specific library
prefix, path, and file extension, indicates a library whose name is,
for example, L, and a native library called L is statically linked
with the VM, then the JNI_OnLoad_L function exported by the library
is invoked rather than attempting to load a dynamic library.
A filename matching the argument does not have to exist in the file
system.
See the JNI Specification
for more details.
Otherwise, the filename argument is mapped to a native library image in
an implementation-dependent manner.
First, if there is a security manager, its checkLink
method is called with the filename
as its argument.
This may result in a security exception.
This is similar to the method loadLibrary(java.lang.String)
, but it
accepts a general file name as an argument rather than just a library
name, allowing any file of native code to be loaded.
The method System#load(String)
is the conventional and
convenient means of invoking this method.
Parameters | |
---|---|
filename |
String : the file to load. |
Throws | |
---|---|
SecurityException |
if a security manager exists and its
checkLink method doesn't allow
loading of the specified dynamic library |
UnsatisfiedLinkError |
if either the filename is not an absolute path name, the native library is not statically linked with the VM, or the library cannot be mapped to a native library image by the host system. |
NullPointerException |
if filename is
null |
loadLibrary
public void loadLibrary (String libname)
Loads the native library specified by the libname
argument. The libname
argument must not contain any platform
specific prefix, file extension or path. If a native library
called libname
is statically linked with the VM, then the
JNI_OnLoad_libname
function exported by the library is invoked.
See the JNI Specification
for more details.
Otherwise, the libname argument is loaded from a system library
location and mapped to a native library image in an implementation-
dependent manner.
First, if there is a security manager, its checkLink
method is called with the libname
as its argument.
This may result in a security exception.
The method System#loadLibrary(String)
is the conventional
and convenient means of invoking this method. If native
methods are to be used in the implementation of a class, a standard
strategy is to put the native code in a library file (call it
LibFile
) and then to put a static initializer:
within the class declaration. When the class is loaded and initialized, the necessary native code implementation for the native methods will then be loaded as well. static { System.loadLibrary("LibFile"); }
If this method is called more than once with the same library name, the second and subsequent calls are ignored.
Parameters | |
---|---|
libname |
String : the name of the library. |
Throws | |
---|---|
SecurityException |
if a security manager exists and its
checkLink method doesn't allow
loading of the specified dynamic library |
UnsatisfiedLinkError |
if either the libname argument contains a file path, the native library is not statically linked with the VM, or the library cannot be mapped to a native library image by the host system. |
NullPointerException |
if libname is
null |
maxMemory
public long maxMemory ()
Returns the maximum amount of memory that the Java virtual machine
will attempt to use. If there is no inherent limit then the value
Long.MAX_VALUE
will be returned.
Returns | |
---|---|
long |
the maximum amount of memory that the virtual machine will attempt to use, measured in bytes |
removeShutdownHook
public boolean removeShutdownHook (Thread hook)
De-registers a previously-registered virtual-machine shutdown hook.
Parameters | |
---|---|
hook |
Thread : the hook to remove |
Returns | |
---|---|
boolean |
true if the specified hook had previously been
registered and was successfully de-registered, false
otherwise. |
Throws | |
---|---|
IllegalStateException |
If the virtual machine is already in the process of shutting down |
SecurityException |
If a security manager is present and it denies
RuntimePermission ("shutdownHooks") |
See also:
runFinalization
public void runFinalization ()
Runs the finalization methods of any objects pending finalization.
Calling this method suggests that the Java virtual machine expend
effort toward running the finalize
methods of objects
that have been found to be discarded but whose finalize
methods have not yet been run. When control returns from the
method call, the virtual machine has made a best effort to
complete all outstanding finalizations.
The virtual machine performs the finalization process
automatically as needed, in a separate thread, if the
runFinalization
method is not invoked explicitly.
The method System#runFinalization()
is the conventional
and convenient means of invoking this method.
See also:
runFinalizersOnExit
public static void runFinalizersOnExit (boolean value)
This method was deprecated
in API level 15.
This method is inherently unsafe. It may result in
finalizers being called on live objects while other threads are
concurrently manipulating those objects, resulting in erratic
behavior or deadlock.
Enable or disable finalization on exit; doing so specifies that the finalizers of all objects that have finalizers that have not yet been automatically invoked are to be run before the Java runtime exits. By default, finalization on exit is disabled.
If there is a security manager,
its checkExit
method is first called
with 0 as its argument to ensure the exit is allowed.
This could result in a SecurityException.
Parameters | |
---|---|
value |
boolean : true to enable finalization on exit, false to disable |
Throws | |
---|---|
SecurityException |
if a security manager exists and its checkExit
method doesn't allow the exit. |
totalMemory
public long totalMemory ()
Returns the total amount of memory in the Java virtual machine. The value returned by this method may vary over time, depending on the host environment.
Note that the amount of memory required to hold an object of any given type may be implementation-dependent.
Returns | |
---|---|
long |
the total amount of memory currently available for current and future objects, measured in bytes. |
traceInstructions
public void traceInstructions (boolean on)
This method was deprecated
in API level 34.
This method was intended to control instruction tracing.
It has been superseded by ART-specific tracing mechanisms.
Not implemented, does nothing.
Parameters | |
---|---|
on |
boolean : ignored |
traceMethodCalls
public void traceMethodCalls (boolean on)
This method was deprecated
in API level 34.
This method was intended to control method call tracing.
It has been superseded by ART-specific tracing mechanisms.
Not implemented, does nothing.
Calling this method on Android Lollipop or later (API level >= 21)
with true
argument will cause it to throw an
UnsupportedOperationException
.
Parameters | |
---|---|
on |
boolean : ignored |