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javalang is a pure Python library for working with Java source
code. javalang provides a lexer and parser targeting Java 8. The
implementation is based on the Java language spec available at
The following gives a very brief introduction to using javalang.
Getting Started
.. code-block:: python
>>> import javalang
>>> tree = javalang.parse.parse("package; class Test {}")
This will return a ``CompilationUnit`` instance. This object is the root of a
tree which may be traversed to extract different information about the
compilation unit,
.. code-block:: python
>>> tree.types[0]
>>> tree.types[0].name
The string passed to ``javalang.parse.parse()`` must represent a complete unit
which simply means it should represent a complete, valid Java source file. Other
methods in the ``javalang.parse`` module allow for some smaller code snippets to
be parsed without providing an entire compilation unit.
Working with the syntax tree
``CompilationUnit`` is a subclass of ``javalang.ast.Node``, as are its
descendants in the tree. The ``javalang.tree`` module defines the different
types of ``Node`` subclasses, each of which represent the different syntaxual
elements you will find in Java code. For more detail on what node types are
available, see the ``javalang/`` source file until the documentation is
``Node`` instances support iteration,
.. code-block:: python
>>> for path, node in tree:
... print path, node
() CompilationUnit
(CompilationUnit,) PackageDeclaration
(CompilationUnit, [ClassDeclaration]) ClassDeclaration
This iteration can also be filtered by type,
.. code-block:: python
>>> for path, node in tree.filter(javalang.tree.ClassDeclaration):
... print path, node
(CompilationUnit, [ClassDeclaration]) ClassDeclaration
Component Usage
Internally, the ``javalang.parse.parse`` method is a simple method which creates
a token stream for the input, initializes a new ``javalang.parser.Parser``
instance with the given token stream, and then invokes the parser's ``parse()``
method, returning the resulting ``CompilationUnit``. These components may be
also be used individually.
The tokenizer/lexer may be invoked directly be calling ``javalang.tokenizer.tokenize``,
.. code-block:: python
>>> javalang.tokenizer.tokenize('System.out.println("Hello " + "world");')
<generator object tokenize at 0x1ce5190>
This returns a generator which provides a stream of ``JavaToken`` objects. Each
token carries position (line, column) and value information,
.. code-block:: python
>>> tokens = list(javalang.tokenizer.tokenize('System.out.println("Hello " + "world");'))
>>> tokens[6].value
u'"Hello "'
>>> tokens[6].position
(1, 19)
The tokens are not directly instances of ``JavaToken``, but are instead
instances of subclasses which identify their general type,
.. code-block:: python
>>> type(tokens[6])
<class 'javalang.tokenizer.String'>
>>> type(tokens[7])
<class 'javalang.tokenizer.Operator'>
**NOTE:** The shift operators ``>>`` and ``>>>`` are represented by multiple
``>`` tokens. This is because multiple ``>`` may appear in a row when closing
nested generic parameter/arguments lists. This abiguity is instead resolved by
the parser.
To parse snippets of code, a parser may be used directly,
.. code-block:: python
>>> tokens = javalang.tokenizer.tokenize('System.out.println("Hello " + "world");')
>>> parser = javalang.parser.Parser(tokens)
>>> parser.parse_expression()
The parse methods are designed for incremental parsing so they will not restart
at the beginning of the token stream. Attempting to call a parse method more
than once will result in a ``JavaSyntaxError`` exception.
Invoking the incorrect parse method will also result in a ``JavaSyntaxError``
.. code-block:: python
>>> tokens = javalang.tokenizer.tokenize('System.out.println("Hello " + "world");')
>>> parser = javalang.parser.Parser(tokens)
>>> parser.parse_type_declaration()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "javalang/", line 336, in parse_type_declaration
return self.parse_class_or_interface_declaration()
File "javalang/", line 353, in parse_class_or_interface_declaration
self.illegal("Expected type declaration")
File "javalang/", line 122, in illegal
raise JavaSyntaxError(description, at)
The ``javalang.parse`` module also provides convenience methods for parsing more
common types of code snippets.