Class Process
Process
provides control of native processes started by
ProcessBuilder.start and Runtime.exec.
The class provides methods for performing input from the process, performing
output to the process, waiting for the process to complete,
checking the exit status of the process, and destroying (killing)
the process.
The ProcessBuilder.start()
and
Runtime.exec
methods create a native process and return an instance of a
subclass of Process
that can be used to control the process
and obtain information about it.
The methods that create processes may not work well for special processes on certain native platforms, such as native windowing processes, daemon processes, Win16/DOS processes on Microsoft Windows, or shell scripts.
By default, the created process does not have its own terminal
or console. All its standard I/O (i.e. stdin, stdout, stderr)
operations will be redirected to the parent process, where they can
be accessed via the streams obtained using the methods
getOutputStream()
,
getInputStream()
, and
getErrorStream()
.
The I/O streams of characters and lines can be written and read using the methods
outputWriter()
, outputWriter(Charset)
,
inputReader()
, inputReader(Charset)
,
errorReader()
, and errorReader(Charset)
.
The parent process uses these streams to feed input to and get output
from the process. Because some native platforms only provide
limited buffer size for standard input and output streams, failure
to promptly write the input stream or read the output stream of
the process may cause the process to block, or even deadlock.
Where desired,
process I/O can also be redirected
using methods of the ProcessBuilder
class.
The process is not killed when there are no more references to
the Process
object, but rather the process
continues executing asynchronously.
There is no requirement that the process represented by a
Process
object execute asynchronously or concurrently with respect
to the Java process that owns the Process
object.
As of 1.5, ProcessBuilder.start()
is the preferred way
to create a Process
.
Subclasses of Process should override the onExit()
and
toHandle()
methods to provide a fully functional Process including the
process id,
information about the process,
direct children, and
direct children plus descendants of those children of the process.
Delegating to the underlying Process or ProcessHandle is typically
easiest and most efficient.
- Since:
- 1.0
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Constructor Summary
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Method Summary
Modifier and TypeMethodDescriptionchildren()
Returns a snapshot of the direct children of the process.Returns a snapshot of the descendants of the process.abstract void
destroy()
Kills the process.Kills the process forcibly.final BufferedReader
Returns aBufferedReader
connected to the standard error of the process.final BufferedReader
errorReader
(Charset charset) Returns aBufferedReader
connected to the standard error of this process using a Charset.abstract int
Returns the exit value for the process.abstract InputStream
Returns the input stream connected to the error output of the process.abstract InputStream
Returns the input stream connected to the normal output of the process.abstract OutputStream
Returns the output stream connected to the normal input of the process.info()
Returns a snapshot of information about the process.final BufferedReader
Returns aBufferedReader
connected to the standard output of the process.final BufferedReader
inputReader
(Charset charset) Returns aBufferedReader
connected to the standard output of this process using a Charset.boolean
isAlive()
Tests whether the process represented by thisProcess
is alive.onExit()
Returns aCompletableFuture<Process>
for the termination of the Process.final BufferedWriter
Returns aBufferedWriter
connected to the normal input of the process using the native encoding.final BufferedWriter
outputWriter
(Charset charset) Returns aBufferedWriter
connected to the normal input of the process using a Charset.long
pid()
Returns the native process ID of the process.boolean
Returnstrue
if the implementation ofdestroy()
is to normally terminate the process, Returnsfalse
if the implementation ofdestroy
forcibly and immediately terminates the process.toHandle()
Returns a ProcessHandle for the Process.abstract int
waitFor()
Causes the current thread to wait, if necessary, until the process represented by thisProcess
object has terminated.boolean
Causes the current thread to wait, if necessary, until the process represented by thisProcess
object has terminated, or the specified waiting time elapses.boolean
Causes the current thread to wait, if necessary, until the process represented by thisProcess
object has terminated, or the specified waiting duration elapses.
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Constructor Details
-
Process
public Process()Default constructor for Process.
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Method Details
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getOutputStream
Returns the output stream connected to the normal input of the process. Output to the stream is piped into the standard input of the process represented by thisProcess
object.If the standard input of the process has been redirected using
ProcessBuilder.redirectInput
then this method will return a null output stream.- API Note:
- When writing to both
getOutputStream()
and eitheroutputWriter()
oroutputWriter(Charset)
,BufferedWriter.flush
should be called before writes to theOutputStream
. - Implementation Note:
- Implementation note: It is a good idea for the returned output stream to be buffered.
- Returns:
- the output stream connected to the normal input of the process
-
getInputStream
Returns the input stream connected to the normal output of the process. The stream obtains data piped from the standard output of the process represented by thisProcess
object.If the standard output of the process has been redirected using
ProcessBuilder.redirectOutput
then this method will return a null input stream.Otherwise, if the standard error of the process has been redirected using
ProcessBuilder.redirectErrorStream
then the input stream returned by this method will receive the merged standard output and the standard error of the process.- API Note:
- Use
getInputStream()
andinputReader()
with extreme care. TheBufferedReader
may have buffered input from the input stream. - Implementation Note:
- Implementation note: It is a good idea for the returned input stream to be buffered.
- Returns:
- the input stream connected to the normal output of the process
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getErrorStream
Returns the input stream connected to the error output of the process. The stream obtains data piped from the error output of the process represented by thisProcess
object.If the standard error of the process has been redirected using
ProcessBuilder.redirectError
orProcessBuilder.redirectErrorStream
then this method will return a null input stream.- API Note:
- Use
getErrorStream()
anderrorReader()
with extreme care. TheBufferedReader
may have buffered input from the error stream. - Implementation Note:
- Implementation note: It is a good idea for the returned input stream to be buffered.
- Returns:
- the input stream connected to the error output of the process
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inputReader
Returns aBufferedReader
connected to the standard output of the process. TheCharset
for the native encoding is used to read characters, lines, or stream lines from standard output.This method delegates to
inputReader(Charset)
using theCharset
named by thenative.encoding
system property. If thenative.encoding
is not a valid charset name or not supported theCharset.defaultCharset()
is used.- Returns:
- a
BufferedReader
using thenative.encoding
if supported, otherwise, theCharset.defaultCharset()
- Since:
- 17
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inputReader
Returns aBufferedReader
connected to the standard output of this process using a Charset. TheBufferedReader
can be used to read characters, lines, or stream lines of the standard output.Characters are read by an InputStreamReader that reads and decodes bytes from this process
getInputStream()
. Bytes are decoded to characters using thecharset
; malformed-input and unmappable-character sequences are replaced with the charset's default replacement. TheBufferedReader
reads and buffers characters from the InputStreamReader.The first call to this method creates the
BufferedReader
, if called again with the samecharset
the sameBufferedReader
is returned. It is an error to call this method again with a differentcharset
.If the standard output of the process has been redirected using
ProcessBuilder.redirectOutput
then theInputStreamReader
will be reading from a null input stream.Otherwise, if the standard error of the process has been redirected using
ProcessBuilder.redirectErrorStream
then the input reader returned by this method will receive the merged standard output and the standard error of the process.- API Note:
- Using both
getInputStream()
andinputReader(Charset)
has unpredictable behavior since the buffered reader reads ahead from the input stream.When the process has terminated, and the standard input has not been redirected, reading of the bytes available from the underlying stream is on a best effort basis and may be unpredictable.
- Parameters:
charset
- theCharset
used to decode bytes to characters- Returns:
- a
BufferedReader
for the standard output of the process using thecharset
- Throws:
NullPointerException
- if thecharset
isnull
IllegalStateException
- if called more than once with different charset arguments- Since:
- 17
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errorReader
Returns aBufferedReader
connected to the standard error of the process. TheCharset
for the native encoding is used to read characters, lines, or stream lines from standard error.This method delegates to
errorReader(Charset)
using theCharset
named by thenative.encoding
system property. If thenative.encoding
is not a valid charset name or not supported theCharset.defaultCharset()
is used.- Returns:
- a
BufferedReader
using thenative.encoding
if supported, otherwise, theCharset.defaultCharset()
- Since:
- 17
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errorReader
Returns aBufferedReader
connected to the standard error of this process using a Charset. TheBufferedReader
can be used to read characters, lines, or stream lines of the standard error.Characters are read by an InputStreamReader that reads and decodes bytes from this process
getErrorStream()
. Bytes are decoded to characters using thecharset
; malformed-input and unmappable-character sequences are replaced with the charset's default replacement. TheBufferedReader
reads and buffers characters from the InputStreamReader.The first call to this method creates the
BufferedReader
, if called again with the samecharset
the sameBufferedReader
is returned. It is an error to call this method again with a differentcharset
.If the standard error of the process has been redirected using
ProcessBuilder.redirectError
orProcessBuilder.redirectErrorStream
then theInputStreamReader
will be reading from a null input stream.- API Note:
- Using both
getErrorStream()
anderrorReader(Charset)
has unpredictable behavior since the buffered reader reads ahead from the error stream.When the process has terminated, and the standard error has not been redirected, reading of the bytes available from the underlying stream is on a best effort basis and may be unpredictable.
- Parameters:
charset
- theCharset
used to decode bytes to characters- Returns:
- a
BufferedReader
for the standard error of the process using thecharset
- Throws:
NullPointerException
- if thecharset
isnull
IllegalStateException
- if called more than once with different charset arguments- Since:
- 17
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outputWriter
Returns aBufferedWriter
connected to the normal input of the process using the native encoding. Writes text to a character-output stream, buffering characters so as to provide for the efficient writing of single characters, arrays, and strings.This method delegates to
outputWriter(Charset)
using theCharset
named by thenative.encoding
system property. If thenative.encoding
is not a valid charset name or not supported theCharset.defaultCharset()
is used.- Returns:
- a
BufferedWriter
to the standard input of the process using the charset for thenative.encoding
system property - Since:
- 17
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outputWriter
Returns aBufferedWriter
connected to the normal input of the process using a Charset. Writes text to a character-output stream, buffering characters so as to provide for the efficient writing of single characters, arrays, and strings.Characters written by the writer are encoded to bytes using
OutputStreamWriter
and theCharset
are written to the standard input of the process represented by thisProcess
. Malformed-input and unmappable-character sequences are replaced with the charset's default replacement.The first call to this method creates the
BufferedWriter
, if called again with the samecharset
the sameBufferedWriter
is returned. It is an error to call this method again with a differentcharset
.If the standard input of the process has been redirected using
ProcessBuilder.redirectInput
then theOutputStreamWriter
writes to a null output stream.- API Note:
- A BufferedWriter writes characters, arrays of characters, and strings.
Wrapping the
BufferedWriter
with aPrintWriter
provides efficient buffering and formatting of primitives and objects as well as support for auto-flush on line endings. Call theBufferedWriter.flush()
method to flush buffered output to the process.When writing to both
getOutputStream()
and eitheroutputWriter()
oroutputWriter(Charset)
, BufferedWriter.flush should be called before writes to theOutputStream
. - Parameters:
charset
- theCharset
to encode characters to bytes- Returns:
- a
BufferedWriter
to the standard input of the process using thecharset
- Throws:
NullPointerException
- if thecharset
isnull
IllegalStateException
- if called more than once with different charset arguments- Since:
- 17
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waitFor
Causes the current thread to wait, if necessary, until the process represented by thisProcess
object has terminated. This method returns immediately if the process has already terminated. If the process has not yet terminated, the calling thread will be blocked until the process exits.- Returns:
- the exit value of the process represented by this
Process
object. By convention, the value0
indicates normal termination. - Throws:
InterruptedException
- if the current thread is interrupted by another thread while it is waiting, then the wait is ended and anInterruptedException
is thrown.
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waitFor
Causes the current thread to wait, if necessary, until the process represented by thisProcess
object has terminated, or the specified waiting time elapses.If the process has already terminated then this method returns immediately with the value
true
. If the process has not terminated and the timeout value is less than, or equal to, zero, then this method returns immediately with the valuefalse
.- Implementation Requirements:
- The default implementation of this method polls the
exitValue
to check if the process has terminated. - Implementation Note:
- Concrete implementations of this class are strongly encouraged to override this method with a more efficient implementation.
- Parameters:
timeout
- the maximum time to waitunit
- the time unit of thetimeout
argument- Returns:
true
if the process has exited andfalse
if the waiting time elapsed before the process has exited.- Throws:
InterruptedException
- if the current thread is interrupted while waiting.NullPointerException
- if unit is null- Since:
- 1.8
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waitFor
Causes the current thread to wait, if necessary, until the process represented by thisProcess
object has terminated, or the specified waiting duration elapses.If the process has already terminated then this method returns immediately with the value
true
. If the process has not terminated and the duration is not positive, then this method returns immediately with the valuefalse
.- Implementation Requirements:
- The default implementation of this method polls the
exitValue
to check if the process has terminated. - Implementation Note:
- Concrete implementations of this class are strongly encouraged to override this method with a more efficient implementation.
- Parameters:
duration
- the maximum duration to wait; if not positive, this method returns immediately.- Returns:
true
if the process has exited andfalse
if the waiting duration elapsed before the process has exited.- Throws:
InterruptedException
- if the current thread is interrupted while waiting.NullPointerException
- if duration is null- Since:
- 24
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exitValue
public abstract int exitValue()Returns the exit value for the process.- Returns:
- the exit value of the process represented by this
Process
object. By convention, the value0
indicates normal termination. - Throws:
IllegalThreadStateException
- if the process represented by thisProcess
object has not yet terminated
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destroy
public abstract void destroy()Kills the process. Whether the process represented by thisProcess
object is normally terminated or not is implementation dependent. Forcible process destruction is defined as the immediate termination of a process, whereas normal termination allows the process to shut down cleanly. If the process is not alive, no action is taken.The
CompletableFuture
fromonExit()
is completed when the process has terminated. -
destroyForcibly
Kills the process forcibly. The process represented by thisProcess
object is forcibly terminated. Forcible process destruction is defined as the immediate termination of a process, whereas normal termination allows the process to shut down cleanly. If the process is not alive, no action is taken.The
CompletableFuture
fromonExit()
is completed when the process has terminated.Invoking this method on
Process
objects returned byProcessBuilder.start()
andRuntime.exec(java.lang.String)
forcibly terminate the process.- API Note:
- The process may not terminate immediately.
i.e.
isAlive()
may return true for a brief period afterdestroyForcibly()
is called. This method may be chained towaitFor()
if needed. - Implementation Requirements:
- The default implementation of this method invokes
destroy()
and so may not forcibly terminate the process. - Implementation Note:
- Concrete implementations of this class are strongly encouraged to override this method with a compliant implementation.
- Returns:
- the
Process
object representing the process forcibly destroyed - Since:
- 1.8
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supportsNormalTermination
public boolean supportsNormalTermination()Returnstrue
if the implementation ofdestroy()
is to normally terminate the process, Returnsfalse
if the implementation ofdestroy
forcibly and immediately terminates the process.Invoking this method on
Process
objects returned byProcessBuilder.start()
andRuntime.exec(java.lang.String)
returntrue
orfalse
depending on the platform implementation.- Implementation Requirements:
- This implementation throws an instance of
UnsupportedOperationException
and performs no other action. - Returns:
true
if the implementation ofdestroy()
is to normally terminate the process; otherwise,destroy()
forcibly terminates the process- Throws:
UnsupportedOperationException
- if the Process implementation does not support this operation- Since:
- 9
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isAlive
public boolean isAlive()Tests whether the process represented by thisProcess
is alive.- Returns:
true
if the process represented by thisProcess
object has not yet terminated.- Since:
- 1.8
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pid
public long pid()Returns the native process ID of the process. The native process ID is an identification number that the operating system assigns to the process.- Implementation Requirements:
- The implementation of this method returns the process id as:
toHandle().pid()
. - Returns:
- the native process id of the process
- Throws:
UnsupportedOperationException
- if the Process implementation does not support this operation- Since:
- 9
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onExit
Returns aCompletableFuture<Process>
for the termination of the Process. TheCompletableFuture
provides the ability to trigger dependent functions or actions that may be run synchronously or asynchronously upon process termination. When the process has terminated the CompletableFuture iscompleted
regardless of the exit status of the process.Calling
onExit().get()
waits for the process to terminate and returns the Process. The future can be used to check if the process is done or to wait for it to terminate. Cancelling the CompletableFuture does not affect the Process.Processes returned from
ProcessBuilder.start()
override the default implementation to provide an efficient mechanism to wait for process exit.- API Note:
- Using
onExit
is an alternative towaitFor
that enables both additional concurrency and convenient access to the result of the Process. Lambda expressions can be used to evaluate the result of the Process execution. If there is other processing to be done before the value is used then onExit is a convenient mechanism to free the current thread and block only if and when the value is needed.
For example, launching a process to compare two files and get a boolean if they are identical:Process p = new ProcessBuilder("cmp", "f1", "f2").start(); Future<Boolean> identical = p.onExit().thenApply(p1 -> p1.exitValue() == 0); ... if (identical.get()) { ... }
isAlive()
before the ComputableFuture is completed and dependent actions are invoked. - Implementation Requirements:
- This implementation executes
waitFor()
in a separate thread repeatedly until it returns successfully. If the execution ofwaitFor
is interrupted, the thread's interrupt status is preserved.When
waitFor()
returns successfully the CompletableFuture is completed regardless of the exit status of the process. This implementation may consume a lot of memory for thread stacks if a large number of processes are waited for concurrently.External implementations should override this method and provide a more efficient implementation. For example, to delegate to the underlying process, it can do the following:
public CompletableFuture<Process> onExit() { return delegate.onExit().thenApply(p -> this); }
- Returns:
- a new
CompletableFuture<Process>
for the Process - Since:
- 9
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toHandle
Returns a ProcessHandle for the Process.Process
objects returned byProcessBuilder.start()
andRuntime.exec(java.lang.String)
implementtoHandle
as the equivalent ofProcessHandle.of(pid)
including the check for a SecurityManager andRuntimePermission("manageProcess")
.- Implementation Requirements:
- This implementation throws an instance of
UnsupportedOperationException
and performs no other action. Subclasses should override this method to provide a ProcessHandle for the process. The methodspid()
,info()
,children()
, anddescendants()
, unless overridden, operate on the ProcessHandle. - Returns:
- Returns a ProcessHandle for the Process
- Throws:
UnsupportedOperationException
- if the Process implementation does not support this operationSecurityException
- if a security manager has been installed and it denies RuntimePermission("manageProcess")- Since:
- 9
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info
Returns a snapshot of information about the process.A
ProcessHandle.Info
instance has accessor methods that return information about the process if it is available.- Implementation Requirements:
- This implementation returns information about the process as:
toHandle().info()
. - Returns:
- a snapshot of information about the process, always non-null
- Throws:
UnsupportedOperationException
- if the Process implementation does not support this operation- Since:
- 9
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children
Returns a snapshot of the direct children of the process. The parent of a direct child process is the process. Typically, a process that is not alive has no children.Note that processes are created and terminate asynchronously. There is no guarantee that a process is alive.
- Implementation Requirements:
- This implementation returns the direct children as:
toHandle().children()
. - Returns:
- a sequential Stream of ProcessHandles for processes that are direct children of the process
- Throws:
UnsupportedOperationException
- if the Process implementation does not support this operationSecurityException
- if a security manager has been installed and it denies RuntimePermission("manageProcess")- Since:
- 9
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descendants
Returns a snapshot of the descendants of the process. The descendants of a process are the children of the process plus the descendants of those children, recursively. Typically, a process that is not alive has no children.Note that processes are created and terminate asynchronously. There is no guarantee that a process is alive.
- Implementation Requirements:
- This implementation returns all children as:
toHandle().descendants()
. - Returns:
- a sequential Stream of ProcessHandles for processes that are descendants of the process
- Throws:
UnsupportedOperationException
- if the Process implementation does not support this operationSecurityException
- if a security manager has been installed and it denies RuntimePermission("manageProcess")- Since:
- 9
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