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ProcessBuilder

public final class ProcessBuilder
extends Object

java.lang.Object
   ↳ java.lang.ProcessBuilder

This class is used to create operating system processes.

Each ProcessBuilder instance manages a collection of process attributes. The start() method creates a new Process instance with those attributes. The start() method can be invoked repeatedly from the same instance to create new subprocesses with identical or related attributes.

Each process builder manages these process attributes:

  • a command, a list of strings which signifies the external program file to be invoked and its arguments, if any. Which string lists represent a valid operating system command is system-dependent. For example, it is common for each conceptual argument to be an element in this list, but there are operating systems where programs are expected to tokenize command line strings themselves - on such a system a Java implementation might require commands to contain exactly two elements.
  • an environment, which is a system-dependent mapping from variables to values. The initial value is a copy of the environment of the current process (see System#getenv()).
  • a working directory. The default value is the current working directory of the current process, usually the directory named by the system property user.dir.
  • a source of standard input. By default, the subprocess reads input from a pipe. Java code can access this pipe via the output stream returned by Process#getOutputStream(). However, standard input may be redirected to another source using redirectInput. In this case, Process#getOutputStream() will return a null output stream, for which:
    • the write methods always throw IOException
    • the close method does nothing
  • a destination for standard output and standard error. By default, the subprocess writes standard output and standard error to pipes. Java code can access these pipes via the input streams returned by Process#getInputStream() and Process#getErrorStream(). However, standard output and standard error may be redirected to other destinations using redirectOutput and redirectError. In this case, Process#getInputStream() and/or Process#getErrorStream() will return a null input stream, for which:
    • the read methods always return -1
    • the available method always returns 0
    • the close method does nothing
  • a redirectErrorStream property. Initially, this property is false, meaning that the standard output and error output of a subprocess are sent to two separate streams, which can be accessed using the Process#getInputStream() and Process.getErrorStream() methods.

    If the value is set to true, then:

    • standard error is merged with the standard output and always sent to the same destination (this makes it easier to correlate error messages with the corresponding output)
    • the common destination of standard error and standard output can be redirected using redirectOutput
    • any redirection set by the redirectError method is ignored when creating a subprocess
    • the stream returned from Process#getErrorStream() will always be a null input stream

Modifying a process builder's attributes will affect processes subsequently started by that object's start() method, but will never affect previously started processes or the Java process itself.

Most error checking is performed by the start() method. It is possible to modify the state of an object so that start() will fail. For example, setting the command attribute to an empty list will not throw an exception unless start() is invoked.

Note that this class is not synchronized. If multiple threads access a ProcessBuilder instance concurrently, and at least one of the threads modifies one of the attributes structurally, it must be synchronized externally.

Starting a new process which uses the default working directory and environment is easy: 

 Process p = new ProcessBuilder("myCommand", "myArg").start();

Here is an example that starts a process with a modified working directory and environment, and redirects standard output and error to be appended to a log file: 

 ProcessBuilder pb =
   new ProcessBuilder("myCommand", "myArg1", "myArg2");
 Map<String, String> env = pb.environment();
 env.put("VAR1", "myValue");
 env.remove("OTHERVAR");
 env.put("VAR2", env.get("VAR1") + "suffix");
 pb.directory(new File("myDir"));
 File log = new File("log");
 pb.redirectErrorStream(true);
 pb.redirectOutput(Redirect.appendTo(log));
 Process p = pb.start();
 assert pb.redirectInput() == Redirect.PIPE;
 assert pb.redirectOutput().file() == log;
 assert p.getInputStream().read() == -1;

To start a process with an explicit set of environment variables, first call Map.clear() before adding environment variables.

Summary

Nested classes

class ProcessBuilder.Redirect

Represents a source of subprocess input or a destination of subprocess output. 

Public constructors

ProcessBuilder(List<String> command)

Constructs a process builder with the specified operating system program and arguments.

ProcessBuilder(String... command)

Constructs a process builder with the specified operating system program and arguments.

Public methods

List<String> command()

Returns this process builder's operating system program and arguments.

ProcessBuilder command(String... command)

Sets this process builder's operating system program and arguments.

ProcessBuilder command(List<String> command)

Sets this process builder's operating system program and arguments.

ProcessBuilder directory(File directory)

Sets this process builder's working directory.

File directory()

Returns this process builder's working directory.

Map<String, String> environment()

Returns a string map view of this process builder's environment.

ProcessBuilder inheritIO()

Sets the source and destination for subprocess standard I/O to be the same as those of the current Java process.

ProcessBuilder redirectError(ProcessBuilder.Redirect destination)

Sets this process builder's standard error destination.

ProcessBuilder.Redirect redirectError()

Returns this process builder's standard error destination.

ProcessBuilder redirectError(File file)

Sets this process builder's standard error destination to a file.

boolean redirectErrorStream()

Tells whether this process builder merges standard error and standard output.

ProcessBuilder redirectErrorStream(boolean redirectErrorStream)

Sets this process builder's redirectErrorStream property.

ProcessBuilder redirectInput(File file)

Sets this process builder's standard input source to a file.

ProcessBuilder.Redirect redirectInput()

Returns this process builder's standard input source.

ProcessBuilder redirectInput(ProcessBuilder.Redirect source)

Sets this process builder's standard input source.

ProcessBuilder redirectOutput(ProcessBuilder.Redirect destination)

Sets this process builder's standard output destination.

ProcessBuilder.Redirect redirectOutput()

Returns this process builder's standard output destination.

ProcessBuilder redirectOutput(File file)

Sets this process builder's standard output destination to a file.

Process start()

Starts a new process using the attributes of this process builder.

Inherited methods

Public constructors

ProcessBuilder

public ProcessBuilder (List<String> command)

Constructs a process builder with the specified operating system program and arguments. This constructor does not make a copy of the command list. Subsequent updates to the list will be reflected in the state of the process builder. It is not checked whether command corresponds to a valid operating system command.

Parameters
command List: the list containing the program and its arguments
Throws
NullPointerException if the argument is null

ProcessBuilder

public ProcessBuilder (String... command)

Constructs a process builder with the specified operating system program and arguments. This is a convenience constructor that sets the process builder's command to a string list containing the same strings as the command array, in the same order. It is not checked whether command corresponds to a valid operating system command.

Parameters
command String: a string array containing the program and its arguments

Public methods

command

public List<String> command ()

Returns this process builder's operating system program and arguments. The returned list is not a copy. Subsequent updates to the list will be reflected in the state of this process builder.

Returns
List<String> this process builder's program and its arguments

command

public ProcessBuilder command (String... command)

Sets this process builder's operating system program and arguments. This is a convenience method that sets the command to a string list containing the same strings as the command array, in the same order. It is not checked whether command corresponds to a valid operating system command.

Parameters
command String: a string array containing the program and its arguments
Returns
ProcessBuilder this process builder

command

public ProcessBuilder command (List<String> command)

Sets this process builder's operating system program and arguments. This method does not make a copy of the command list. Subsequent updates to the list will be reflected in the state of the process builder. It is not checked whether command corresponds to a valid operating system command.

Parameters
command List: the list containing the program and its arguments
Returns
ProcessBuilder this process builder
Throws
NullPointerException if the argument is null

directory

public ProcessBuilder directory (File directory)

Sets this process builder's working directory. Subprocesses subsequently started by this object's start() method will use this as their working directory. The argument may be null -- this means to use the working directory of the current Java process, usually the directory named by the system property user.dir, as the working directory of the child process.

Parameters
directory File: the new working directory
Returns
ProcessBuilder this process builder

directory

public File directory ()

Returns this process builder's working directory. Subprocesses subsequently started by this object's start() method will use this as their working directory. The returned value may be null -- this means to use the working directory of the current Java process, usually the directory named by the system property user.dir, as the working directory of the child process.

Returns
File this process builder's working directory

environment

public Map<String, String> environment ()

Returns a string map view of this process builder's environment. Whenever a process builder is created, the environment is initialized to a copy of the current process environment (see System#getenv()). Subprocesses subsequently started by this object's start() method will use this map as their environment.

The returned object may be modified using ordinary Map operations. These modifications will be visible to subprocesses started via the start() method. Two ProcessBuilder instances always contain independent process environments, so changes to the returned map will never be reflected in any other ProcessBuilder instance or the values returned by System.getenv.

If the system does not support environment variables, an empty map is returned.

The returned map does not permit null keys or values. Attempting to insert or query the presence of a null key or value will throw a NullPointerException. Attempting to query the presence of a key or value which is not of type String will throw a ClassCastException.

The behavior of the returned map is system-dependent. A system may not allow modifications to environment variables or may forbid certain variable names or values. For this reason, attempts to modify the map may fail with UnsupportedOperationException or IllegalArgumentException if the modification is not permitted by the operating system.

Since the external format of environment variable names and values is system-dependent, there may not be a one-to-one mapping between them and Java's Unicode strings. Nevertheless, the map is implemented in such a way that environment variables which are not modified by Java code will have an unmodified native representation in the subprocess.

The returned map and its collection views may not obey the general contract of the Object#equals and Object#hashCode methods.

The returned map is typically case-sensitive on all platforms.

If a security manager exists, its checkPermission method is called with a RuntimePermission("getenv.*") permission. This may result in a SecurityException being thrown.

When passing information to a Java subprocess, system properties are generally preferred over environment variables.

Returns
Map<String, String> this process builder's environment
Throws
SecurityException if a security manager exists and its checkPermission method doesn't allow access to the process environment

See also:

inheritIO

public ProcessBuilder inheritIO ()

Sets the source and destination for subprocess standard I/O to be the same as those of the current Java process.

This is a convenience method. An invocation of the form 

 pb.inheritIO()
behaves in exactly the same way as the invocation 
 pb.redirectInput(Redirect.INHERIT)
   .redirectOutput(Redirect.INHERIT)
   .redirectError(Redirect.INHERIT)
This gives behavior equivalent to most operating system command interpreters, or the standard C library function system().

Returns
ProcessBuilder this process builder

redirectError

public ProcessBuilder redirectError (ProcessBuilder.Redirect destination)

Sets this process builder's standard error destination. Subprocesses subsequently started by this object's start() method send their standard error to this destination.

If the destination is Redirect.PIPE (the initial value), then the error output of a subprocess can be read using the input stream returned by Process.getErrorStream(). If the destination is set to any other value, then Process#getErrorStream() will return a null input stream.

If the redirectErrorStream attribute has been set true, then the redirection set by this method has no effect.

Parameters
destination ProcessBuilder.Redirect: the new standard error destination
Returns
ProcessBuilder this process builder
Throws
IllegalArgumentException if the redirect does not correspond to a valid destination of data, that is, has type READ

redirectError

public ProcessBuilder.Redirect redirectError ()

Returns this process builder's standard error destination. Subprocesses subsequently started by this object's start() method redirect their standard error to this destination. The initial value is Redirect.PIPE.

Returns
ProcessBuilder.Redirect this process builder's standard error destination

redirectError

public ProcessBuilder redirectError (File file)

Sets this process builder's standard error destination to a file.

This is a convenience method. An invocation of the form redirectError(file) behaves in exactly the same way as the invocation redirectError (Redirect.to(file)).

Parameters
file File: the new standard error destination
Returns
ProcessBuilder this process builder

redirectErrorStream

public boolean redirectErrorStream ()

Tells whether this process builder merges standard error and standard output.

If this property is true, then any error output generated by subprocesses subsequently started by this object's start() method will be merged with the standard output, so that both can be read using the Process#getInputStream() method. This makes it easier to correlate error messages with the corresponding output. The initial value is false.

Returns
boolean this process builder's redirectErrorStream property

redirectErrorStream

public ProcessBuilder redirectErrorStream (boolean redirectErrorStream)

Sets this process builder's redirectErrorStream property.

If this property is true, then any error output generated by subprocesses subsequently started by this object's start() method will be merged with the standard output, so that both can be read using the Process#getInputStream() method. This makes it easier to correlate error messages with the corresponding output. The initial value is false.

Parameters
redirectErrorStream boolean: the new property value
Returns
ProcessBuilder this process builder

redirectInput

public ProcessBuilder redirectInput (File file)

Sets this process builder's standard input source to a file.

This is a convenience method. An invocation of the form redirectInput(file) behaves in exactly the same way as the invocation redirectInput (Redirect.from(file)).

Parameters
file File: the new standard input source
Returns
ProcessBuilder this process builder

redirectInput

public ProcessBuilder.Redirect redirectInput ()

Returns this process builder's standard input source. Subprocesses subsequently started by this object's start() method obtain their standard input from this source. The initial value is Redirect.PIPE.

Returns
ProcessBuilder.Redirect this process builder's standard input source

redirectInput

public ProcessBuilder redirectInput (ProcessBuilder.Redirect source)

Sets this process builder's standard input source. Subprocesses subsequently started by this object's start() method obtain their standard input from this source.

If the source is Redirect.PIPE (the initial value), then the standard input of a subprocess can be written to using the output stream returned by Process#getOutputStream(). If the source is set to any other value, then Process#getOutputStream() will return a null output stream.

Parameters
source ProcessBuilder.Redirect: the new standard input source
Returns
ProcessBuilder this process builder
Throws
IllegalArgumentException if the redirect does not correspond to a valid source of data, that is, has type WRITE or APPEND

redirectOutput

public ProcessBuilder redirectOutput (ProcessBuilder.Redirect destination)

Sets this process builder's standard output destination. Subprocesses subsequently started by this object's start() method send their standard output to this destination.

If the destination is Redirect.PIPE (the initial value), then the standard output of a subprocess can be read using the input stream returned by Process.getInputStream(). If the destination is set to any other value, then Process#getInputStream() will return a null input stream.

Parameters
destination ProcessBuilder.Redirect: the new standard output destination
Returns
ProcessBuilder this process builder
Throws
IllegalArgumentException if the redirect does not correspond to a valid destination of data, that is, has type READ

redirectOutput

public ProcessBuilder.Redirect redirectOutput ()

Returns this process builder's standard output destination. Subprocesses subsequently started by this object's start() method redirect their standard output to this destination. The initial value is Redirect.PIPE.

Returns
ProcessBuilder.Redirect this process builder's standard output destination

redirectOutput

public ProcessBuilder redirectOutput (File file)

Sets this process builder's standard output destination to a file.

This is a convenience method. An invocation of the form redirectOutput(file) behaves in exactly the same way as the invocation redirectOutput (Redirect.to(file)).

Parameters
file File: the new standard output destination
Returns
ProcessBuilder this process builder

start

public Process start ()

Starts a new process using the attributes of this process builder.

The new process will invoke the command and arguments given by command(), in a working directory as given by directory(), with a process environment as given by environment().

This method checks that the command 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.

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 process builder's environment().

If there is a security manager, its checkExec method is called with the first component of this object's command array 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.

Subsequent modifications to this process builder will not affect the returned Process.

Returns
Process a new Process object for managing the subprocess
Throws
NullPointerException if an element of the command list is null
IndexOutOfBoundsException if the command is an empty list (has size 0)
SecurityException if a security manager exists and
  • its checkExec method doesn't allow creation of the subprocess, or
  • the standard input to the subprocess was redirected from a file and the security manager's checkRead method denies read access to the file, or
  • the standard output or standard error of the subprocess was redirected to a file and the security manager's checkWrite method denies write access to the file
IOException if an I/O error occurs

See also:

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