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
Constructors -
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 voiddestroy()Kills the process.Kills the process forcibly.final BufferedReaderReturns aBufferedReaderconnected to the standard error of the process.final BufferedReadererrorReader(Charset charset) Returns aBufferedReaderconnected to the standard error of this process using a Charset.abstract intReturns the exit value for the process.abstract InputStreamReturns the input stream connected to the error output of the process.abstract InputStreamReturns the input stream connected to the normal output of the process.abstract OutputStreamReturns the output stream connected to the normal input of the process.info()Returns a snapshot of information about the process.final BufferedReaderReturns aBufferedReaderconnected to the standard output of the process.final BufferedReaderinputReader(Charset charset) Returns aBufferedReaderconnected to the standard output of this process using a Charset.booleanisAlive()Tests whether the process represented by thisProcessis alive.onExit()Returns aCompletableFuture<Process>for the termination of the Process.final BufferedWriterReturns aBufferedWriterconnected to the normal input of the process using the native encoding.final BufferedWriteroutputWriter(Charset charset) Returns aBufferedWriterconnected to the normal input of the process using a Charset.longpid()Returns the native process ID of the process.booleanReturnstrueif the implementation ofdestroy()is to normally terminate the process, Returnsfalseif the implementation ofdestroyforcibly and immediately terminates the process.toHandle()Returns a ProcessHandle for the Process.abstract intwaitFor()Causes the current thread to wait, if necessary, until the process represented by thisProcessobject has terminated.booleanCauses the current thread to wait, if necessary, until the process represented by thisProcessobject has terminated, or the specified waiting time elapses.
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Constructor Details
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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 thisProcessobject.If the standard input of the process has been redirected using
ProcessBuilder.redirectInputthen this method will return a null output stream.- API Note:
- When writing to both
getOutputStream()and eitheroutputWriter()oroutputWriter(Charset),BufferedWriter.flushshould 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
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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 thisProcessobject.If the standard output of the process has been redirected using
ProcessBuilder.redirectOutputthen this method will return a null input stream.Otherwise, if the standard error of the process has been redirected using
ProcessBuilder.redirectErrorStreamthen 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. TheBufferedReadermay 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 thisProcessobject.If the standard error of the process has been redirected using
ProcessBuilder.redirectErrororProcessBuilder.redirectErrorStreamthen this method will return a null input stream.- API Note:
- Use
getErrorStream()anderrorReader()with extreme care. TheBufferedReadermay 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 aBufferedReaderconnected to the standard output of the process. TheCharsetfor the native encoding is used to read characters, lines, or stream lines from standard output.This method delegates to
inputReader(Charset)using theCharsetnamed by thenative.encodingsystem property. If thenative.encodingis not a valid charset name or not supported theCharset.defaultCharset()is used.- Returns:
- a
BufferedReaderusing thenative.encodingif supported, otherwise, theCharset.defaultCharset() - Since:
- 17
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inputReader
Returns aBufferedReaderconnected to the standard output of this process using a Charset. TheBufferedReadercan 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. TheBufferedReaderreads and buffers characters from the InputStreamReader.The first call to this method creates the
BufferedReader, if called again with the samecharsetthe sameBufferedReaderis 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.redirectOutputthen theInputStreamReaderwill be reading from a null input stream.Otherwise, if the standard error of the process has been redirected using
ProcessBuilder.redirectErrorStreamthen 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- theCharsetused to decode bytes to characters- Returns:
- a
BufferedReaderfor the standard output of the process using thecharset - Throws:
NullPointerException- if thecharsetisnullIllegalStateException- if called more than once with different charset arguments- Since:
- 17
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errorReader
Returns aBufferedReaderconnected to the standard error of the process. TheCharsetfor the native encoding is used to read characters, lines, or stream lines from standard error.This method delegates to
errorReader(Charset)using theCharsetnamed by thenative.encodingsystem property. If thenative.encodingis not a valid charset name or not supported theCharset.defaultCharset()is used.- Returns:
- a
BufferedReaderusing thenative.encodingif supported, otherwise, theCharset.defaultCharset() - Since:
- 17
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errorReader
Returns aBufferedReaderconnected to the standard error of this process using a Charset. TheBufferedReadercan 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. TheBufferedReaderreads and buffers characters from the InputStreamReader.The first call to this method creates the
BufferedReader, if called again with the samecharsetthe sameBufferedReaderis 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.redirectErrororProcessBuilder.redirectErrorStreamthen theInputStreamReaderwill 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- theCharsetused to decode bytes to characters- Returns:
- a
BufferedReaderfor the standard error of the process using thecharset - Throws:
NullPointerException- if thecharsetisnullIllegalStateException- if called more than once with different charset arguments- Since:
- 17
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outputWriter
Returns aBufferedWriterconnected 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 theCharsetnamed by thenative.encodingsystem property. If thenative.encodingis not a valid charset name or not supported theCharset.defaultCharset()is used.- Returns:
- a
BufferedWriterto the standard input of the process using the charset for thenative.encodingsystem property - Since:
- 17
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outputWriter
Returns aBufferedWriterconnected 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
OutputStreamWriterand theCharsetare 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 samecharsetthe sameBufferedWriteris 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.redirectInputthen theOutputStreamWriterwrites to a null output stream.- API Note:
- A BufferedWriter writes characters, arrays of characters, and strings.
Wrapping the
BufferedWriterwith aPrintWriterprovides 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- theCharsetto encode characters to bytes- Returns:
- a
BufferedWriterto the standard input of the process using thecharset - Throws:
NullPointerException- if thecharsetisnullIllegalStateException- 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 thisProcessobject 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
Processobject. By convention, the value0indicates normal termination. - Throws:
InterruptedException- if the current thread is interrupted by another thread while it is waiting, then the wait is ended and anInterruptedExceptionis thrown.
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waitFor
Causes the current thread to wait, if necessary, until the process represented by thisProcessobject 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.The default implementation of this method polls the
exitValueto check if the process has terminated. 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 thetimeoutargument- Returns:
trueif the process has exited andfalseif 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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exitValue
public abstract int exitValue()Returns the exit value for the process.- Returns:
- the exit value of the process represented by this
Processobject. By convention, the value0indicates normal termination. - Throws:
IllegalThreadStateException- if the process represented by thisProcessobject has not yet terminated
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destroy
public abstract void destroy()Kills the process. Whether the process represented by thisProcessobject 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
CompletableFuturefromonExit()is completed when the process has terminated. -
destroyForcibly
Kills the process forcibly. The process represented by thisProcessobject 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
CompletableFuturefromonExit()is completed when the process has terminated.Invoking this method on
Processobjects 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
Processobject representing the process forcibly destroyed - Since:
- 1.8
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supportsNormalTermination
public boolean supportsNormalTermination()Returnstrueif the implementation ofdestroy()is to normally terminate the process, Returnsfalseif the implementation ofdestroyforcibly and immediately terminates the process.Invoking this method on
Processobjects returned byProcessBuilder.start()andRuntime.exec(java.lang.String)returntrueorfalsedepending on the platform implementation.- Implementation Requirements:
- This implementation throws an instance of
UnsupportedOperationExceptionand performs no other action. - Returns:
trueif 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 thisProcessis alive.- Returns:
trueif the process represented by thisProcessobject 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. TheCompletableFutureprovides 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 iscompletedregardless 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
onExitis an alternative towaitForthat 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:
, The process may be observed to have terminated withProcess 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 ofwaitForis 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.Processobjects returned byProcessBuilder.start()andRuntime.exec(java.lang.String)implementtoHandleas the equivalent ofProcessHandle.of(pid)including the check for a SecurityManager andRuntimePermission("manageProcess").- Implementation Requirements:
- This implementation throws an instance of
UnsupportedOperationExceptionand 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.Infoinstance 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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