Class: java.lang.ProcessBuilder
- public final class ProcessBuilder
This class is used to create operating system processes.
Each ProcessBuilder instance manages a collection
of process attributes. The ProcessBuilder.start() method creates a new
Process instance with those attributes. The ProcessBuilder.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 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 totrue, the standard error is merged with the standard output. This makes it easier to correlate error messages with the corresponding output. In this case, the merged data can be read from the stream returned by Process.getInputStream(), while reading from the stream returned by Process.getErrorStream() will get an immediate end of file.
Modifying a process builder's attributes will affect processes subsequently started by that object's ProcessBuilder.start() method, but will never affect previously started processes or the Java process itself.
Most error checking is performed by the ProcessBuilder.start() method. It is possible to modify the state of an object so that ProcessBuilder.start() will fail. For example, setting the command attribute to an empty list will not throw an exception unless ProcessBuilder.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:
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("myDir");
Process p = pb.start();
To start a process with an explicit set of environment variables, first call Map.clear() before adding environment variables.
Methods
-
ProcessBuildertop
Constructs a process builder with the specified operating system program and arguments. This constructor does not make a copy of thecommandlist. Subsequent updates to the list will be reflected in the state of the process builder. It is not checked whethercommandcorresponds to a valid operating system command. -
ProcessBuildertop
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 thecommandarray, in the same order. It is not checked whethercommandcorresponds to a valid operating system command. -
commandtop
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. -
commandtop
Sets this process builder's operating system program and arguments. This method does not make a copy of thecommandlist. Subsequent updates to the list will be reflected in the state of the process builder. It is not checked whethercommandcorresponds to a valid operating system command. -
commandtop
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 thecommandarray, in the same order. It is not checked whethercommandcorresponds to a valid operating system command. -
directorytop
public File directory()Returns this process builder's working directory. Subprocesses subsequently started by this object's ProcessBuilder.start() method will use this as their working directory. The returned value may benull-- this means to use the working directory of the current Java process, usually the directory named by the system propertyuser.dir, as the working directory of the child process. -
directorytop
public ProcessBuilder directory(File directory)Sets this process builder's working directory. Subprocesses subsequently started by this object's ProcessBuilder.start() method will use this as their working directory. The argument may benull-- this means to use the working directory of the current Java process, usually the directory named by the system propertyuser.dir, as the working directory of the child process. -
environmenttop
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 ProcessBuilder.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 ProcessBuilder.start() method. Two
ProcessBuilderinstances always contain independent process environments, so changes to the returned map will never be reflected in any otherProcessBuilderinstance 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.
-
redirectErrorStreamtop
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 ProcessBuilder.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 isfalse. -
redirectErrorStreamtop
public ProcessBuilder redirectErrorStream(boolean redirectErrorStream)Sets this process builder'sredirectErrorStreamproperty.If this property is
true, then any error output generated by subprocesses subsequently started by this object's ProcessBuilder.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 isfalse. -
starttop
public Process start() throws IOExceptionStarts a new process using the attributes of this process builder.The new process will invoke the command and arguments given by ProcessBuilder.command(), in a working directory as given by ProcessBuilder.directory(), with a process environment as given by ProcessBuilder.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.
If there is a security manager, its checkExec method is called with the first component of this object's
commandarray 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 java.io.IOException.
Subsequent modifications to this process builder will not affect the returned Process.