Jython Registry

Because there is no good platform independent equivalent of the Windows
Registry (or Unix envrionment variables) Java has it’s own environment
variable namespace. Jython aquires it’s namespace from the following
three sources (later sources override defaults found in earlier

  • The Java system properties: typically passed in on the command
    line as options to the java interpreter.

  • The Jython “registry” file, which contains
    prop=value pairs.
    See below for the
    algorithm Jython uses to find the registry file.

  • The user’s personal registry file, which contains
    similarly formated prop/value pairs. The user’s registry file
    is at “user.home“+”/.jython”

  • Jython properties: Specified on the command line as options
    to the jython class. See the -D
    option to the interpreter

Registry Properties

The following properties are recognized by Jython. There may be
others that aren’t documented here; consult the comments in registry
file for details.

Equivalent to CPython’s PYTHONPATH environment variable


The directory to use for caches – currently just package
information. This directory must be writable by the user. If the
directory is an absolute path, it is used as given, otherwise it
is interpreted as relative to sys.prefix.

Sets the verbosity level for varying degrees of informative
messages. Valid values in order of increasing verbosity are
“error”, “warning”, “message”, “comment”, “debug”

Normally, Jython can only provide access to public members of
classes. However if this property is set to false and
you are using Java 1.2 then Jython can access non-public
fields, methods, and constructors.


The Java compiler to use with the jythonc tool,
which now generates Java source code. This should be the absolute
path to a Java compiler, or the name of a compiler on your
standard PATH.

Extensions to the standard java.class.path property
for use with jythonc. This is useful if you use Jikes as your

Options to pass to the Java compiler when using jythonc.


The name of a console class. An alternative console class
that supports
GNU readline

can be installed with this property. Jython already include such a
console class and it can be enabled by setting this property to

Allow a choice of backing implementation for GNU readline support.
Can be either GnuReadline or Editline.
This property is only used when python.console is set to

Finding the Registry File

The following steps are used to find the Jython registry file, and
also to set the Python values for sys.prefix. First a
root directory is calculated:

  • If there is a property called
    python.home, this is used as the root directory.

  • Otherwise, the property install.root is used if it

  • If neither of those properties exist, then Jython searches
    for the file “jython.jar” on the Java classpath, as defined in
    the system property java.class.path. The actual file
    system isn’t searched, only the paths defined on the classpath
    (one of them must literally include “jython.jar”).

Once the root directory is found, sys.prefix and
sys.exec_prefix are set to this, and sys.path has
rootdir/Lib appended to it. The registry file used is then


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