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Java Standard Edition (SE)

ProcessBuilder


compact1, compact2, compact3
java.lang

Class ProcessBuilder



  • public final class ProcessBuilder
    extends Object
    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.

    Since:
    1.5
    • Constructor Detail

      • 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 - 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 - a string array containing the program and its arguments
    • Method Detail

      • 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 - the list containing the program and its arguments
        Returns:
        this process builder
        Throws:
        NullPointerException - if the argument is null
      • 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 - a string array containing the program and its arguments
        Returns:
        this process builder
      • 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:
        this process builder's program and its arguments
      • 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(java.lang.Object) 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:
        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:
        Runtime.exec(String[],String[],java.io.File), System.getenv()
      • 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:
        this process builder's working directory
      • 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 - the new working directory
        Returns:
        this process builder
      • 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 - the new standard output destination
        Returns:
        this process builder
        Throws:
        IllegalArgumentException - if the redirect does not correspond to a valid destination of data, that is, has type READ
        Since:
        1.7
      • 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 - the new standard error destination
        Returns:
        this process builder
        Throws:
        IllegalArgumentException - if the redirect does not correspond to a valid destination of data, that is, has type READ
        Since:
        1.7
      • 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 - the new standard input source
        Returns:
        this process builder
        Since:
        1.7
      • 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 - the new standard output destination
        Returns:
        this process builder
        Since:
        1.7
      • 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 - the new standard error destination
        Returns:
        this process builder
        Since:
        1.7
      • 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:
        this process builder's standard input source
        Since:
        1.7
      • 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:
        this process builder's standard output destination
        Since:
        1.7
      • 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:
        this process builder's standard error destination
        Since:
        1.7
      • 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:
        this process builder
        Since:
        1.7
      • 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:
        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 - the new property value
        Returns:
        this process builder
      • start

        public Process start()
                      throws IOException
        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:
        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:
        Runtime.exec(String[], String[], java.io.File)


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