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<title>JavaCC: JJTree Introduction</title>
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<H1>JavaCC [tm]: JJTree Introduction</H1>
<pre>
JJTree is a preprocessor for JavaCC [tm] that inserts parse tree building actions
at various places in the JavaCC source. The output of JJTree is run through
JavaCC to create the parser. This document describes how to use JJTree, and
how you can interface your parser to it.
By default, JJTree generates code to construct parse tree nodes for each
nonterminal in the language. This behavior can be modified so that some
nonterminals do not have nodes generated, or so that a node is generated for a
part of a production's expansion.
JJTree defines a Java interface Node that all parse tree nodes must
implement. The interface provides methods for operations such as setting the
parent of the node, and for adding children and retrieving them.
JJTree operates in one of two modes, simple and multi (for want of better
terms). In simple mode, each parse tree node is of concrete type SimpleNode; in
multi mode, the type of the parse tree node is derived from the name of the
node. If you don't provide implementations for the node classes JJTree will
generate sample implementations based on SimpleNode for you. You can then
modify the implementations to suit.
Although JavaCC is a top-down parser, JJTree constructs the parse tree from
the bottom up. To do this it uses a stack where it pushes nodes after they
have been created. When it finds a parent for them, it pops the children from
the stack and adds them to the parent, and finally pushes the new parent node
itself. The stack is open, which means that you have access to it from within
grammar actions: you can push, pop and otherwise manipulate its contents
however you feel appropriate. See Node Scopes and User Actions below for more
important information.
JJTree provides decorations for two basic varieties of nodes, and some
syntactic shorthand to make their use convenient.
1.
A definite node is constructed with a specific number of
children. That many nodes are popped from the stack and made the
children of the new node, which is then pushed on the stack
itself. You notate a definite node like this:
#ADefiniteNode(INTEGER EXPRESSION)
A definite node descriptor expression can be any integer expression,
although literal integer constants are by far the most common
expressions.
2.
A conditional node is constructed with all of the children that were
pushed on the stack within its node scope if and only if its condition
evaluates to true. If it evaluates to false, the node is not
constructed, and all of the children remain on the node stack. You
notate a conditional node like this:
#ConditionalNode(BOOLEAN EXPRESSION)
A conditional node descriptor expression can be any boolean
expression. There are two common shorthands for conditional nodes:
1)
Indefinite nodes
#IndefiniteNode is short for #IndefiniteNode(true)
2)
Greater-than nodes
#GTNode(>1) is short for #GTNode(jjtree.arity() > 1)
The indefinite node shorthand (1) can lead to ambiguities in the
JJTree source when it is followed by a parenthesized expansion. In
those cases the shorthand must be replaced by the full expression. For
example:
( ... ) #N ( a() )
is ambiguous; you have to use the explicit condition:
( ... ) #N(true) ( a() )
WARNING: node descriptor expression should not have side-effects. JJTree
doesn't specify how many times the expression will be evaluated.
By default JJTree treats each nonterminal as an indefinite node and derives
the name of the node from the name of its production. You can give it a
different name with the following syntax:
void P1() #MyNode : { ... } { ... }
When the parser recognizes a P1 nonterminal it begins an indefinite node. It
marks the stack, so that any parse tree nodes created and pushed on the stack
by nonterminals in the expansion for P1 will be popped off and made children
of the node MyNode.
If you want to suppress the creation of a node for a production, you can use
the following syntax:
void P2() #void : { ... } { ... }
Now any parse tree nodes pushed by nonterminals in the expansion of P2 will
remain on the stack, to be popped and made children of a production further up
the tree. You can make this the default behavior for non-decorated nodes by
using the NODE_DEFAULT_VOID option.
void P3() : {}
{
P4() ( P5() )+ P6()
}
In this example, an indefinite node P3 is begun, marking the stack, and then a
P4 node, one or more P5 nodes and a P6 node are parsed. Any nodes that they
push are popped and made the children of P3. You can further customize the
generated tree:
void P3() : {}
{
P4() ( P5() )+ #ListOfP5s P6()
}
Now the P3 node will have a P4 node, a ListOfP5s node and a P6 node as
children. The #Name construct acts as a postfix operator, and its scope is the
immediately preceding expansion unit.
Node Scopes and User Actions
Each node is associated with a node scope. User actions within this scope can
access the node under construction by using the special identifier jjtThis to
refer to the node. This identifier is implicitly declared to be of the correct
type for the node, so any fields and methods that the node has can be easily
accessed.
A scope is the expansion unit immediately preceding the node decoration. This
can be a parenthesized expression. When the production signature is decorated
(perhaps implicitly with the default node), the scope is the entire right hand
side of the production including its declaration block.
You can also use an expression involving jjtThis on the left hand side of an
expansion reference. For example:
... ( jjtThis.my_foo = foo() ) #Baz ...
Here jjtThis refers to a Baz node, which has a field called my_foo. The result
of parsing the production foo() is assigned to that my_foo.
The final user action in a node scope is different from all the others. When
the code within it executes, the node's children have already been popped from
the stack and added to the node, which has itself been pushed onto the
stack. The children can now be accessed via the node's methods such as
jjtGetChild().
User actions other than the final one can only access the children on the
stack. They have not yet been added to the node, so they aren't available via
the node's methods.
A conditional node that has a node descriptor expression that evaluates to
false will not get added to the stack, nor have children added to it. The
final user action within a conditional node scope can determine whether the
node was created or not by calling the nodeCreated() method. This returns true
if the node's condition was satisfied and the node was created and pushed on
the node stack, and false otherwise.
Exception handling
An exception thrown by an expansion within a node scope that is not caught
within the node scope is caught by JJTree itself. When this occurs, any nodes
that have been pushed on to the node stack within the node scope are popped
and thrown away. Then the exception is rethrown.
The intention is to make it possible for parsers to implement error recovery
and continue with the node stack in a known state.
WARNING: JJTree currently cannot detect whether exceptions are thrown from
user actions within a node scope. Such an exception will probably be handled
incorrectly.
Node Scope Hooks
If the NODE_SCOPE_HOOK option is set to true, JJTree generates calls to two
user-defined parser methods on the entry and exit of every node scope. The
methods must have the following signatures:
void jjtreeOpenNodeScope(Node n)
void jjtreeCloseNodeScope(Node n)
If the parser is STATIC then these methods will have to be declared as static
as well. They are both called with the current node as a parameter.
One use for these functions is to store the node's first and last tokens so
that the input can be easily reproduced again. For example:
void jjtreeOpenNodeScope(Node n)
{
((MySimpleNode)n).first_token = getToken(1);
}
void jjtreeCloseNodeScope(Node n)
{
((MySimpleNode)n).last_token = getToken(0);
}
where MySimpleNode is based on SimpleNode and has the following additional
fields:
Token first_token, last_token;
Another use might be to store the parser object itself in the node so that
state that should be shared by all nodes produced by that parser can be
provided. For example, the parser might maintain a symbol table.
The Life Cycle of a Node
A node goes through a well determined sequence of steps as it is built. The
following
is that sequence viewed from the perspective of the node itself:
1. the node's constructor is called with a unique integer parameter. This
parameter identifies the kind of node and is especially useful in
simple mode. JJTree automatically generates a file called
parserTreeConstants.java and declares Java constants for the node
identifiers. It also declares an array of Strings called jjtNodeName[]
which maps the identifier integers to the names of the nodes.
2. the node's jjtOpen() method is called.
3. if the option NODE_SCOPE_HOOK is set, the user-defined parser method
openNodeScope() is called and passed the node as its parameter. This
method can initialize fields in the node or call its methods. For
example, it might store the node's first token in the node.
4. if an unhandled exception is thrown while the node is being parsed
then the node is abandoned. JJTree will never refer to it again. It
will not be closed, and the user-defined node scope hook
closeNodeHook() will not be called with it as a parameter.
5. otherwise, if the node is conditional and its conditional expression
evaluates to false then the node is abandoned. It will not be closed,
although the user-defined node scope hook closeNodeHook() might be
called with it as a parameter.
6. otherwise, all of the children of the node as specified by the integer
expression of a definite node, or all the nodes that were pushed on
the stack within a conditional node scope are added to the node. The
order they are added is not specified.
7. the node's jjtClose() method is called.
8. the node is pushed on the stack.
9. if the option NODE_SCOPE_HOOK is set, the user-defined parser method
closenNodeScope() is called and passed the node as its parameter.
10. if the node is not the root node, it is added as a child of another
node and its jjtSetParent() method is called.
Visitor Support
JJTree provides some basic support for the visitor design pattern. If the
VISITOR option is set to true, JJTree will insert an jjtAccept() method into
all of the node classes it generates, and also generate a visitor interface
that can be implemented and passed to the nodes to accept.
The name of the visitor interface is constructed by appending Visitor to the
name of the parser. The interface is regenerated every time that JJTree is
run, so that it accurately represents the set of nodes used by the
parser. This will cause compile time errors if the implementation class has
not been updated for the new nodes. This is a feature.
Options
JJTree 0.3pre4 supports the following options on the command line and in the
JavaCC options statement:
BUILD_NODE_FILES (default: true)
Generate sample implementations for SimpleNode and any other nodes used
in the grammar.
MULTI (default: false)
Generate a multi mode parse tree. The default for this is false,
generating a simple mode parse tree.
NODE_DEFAULT_VOID (default: false)
Instead of making each non-decorated production an indefinite node, make
it void instead.
NODE_FACTORY (default: false)
Use a factory method with following signature to construct nodes:
public static Node jjtCreate(int id)
NODE_PACKAGE (default: "")
The package to generate the node classes into. The default for this is
the parser package.
NODE_PREFIX (default: "AST")
The prefix used to construct node class names from node identifiers in
multi mode. The default for this is AST.
NODE_SCOPE_HOOK (default: false)
Insert calls to user-defined parser methods on entry and exit of every
node scope. See Node Scope Hooks above.
NODE_USES_PARSER (default: false)
JJTree will use an alternate form of the node construction routines
where it passes the parser object in. For example,
public static Node MyNode.jjtCreate(MyParser p, int id);
MyNode(MyParser p, int id);
STATIC (default: true)
Generate code for a static parser. The default for this is true. This
must be used consistently with the equivalent JavaCC options. The value
of this option is emitted in the JavaCC source.
VISITOR (default: false)
Insert a jjtAccept() method in the node classes, and generate a visitor
implementation with an entry for every node type used in the grammar.
VISITOR_EXCEPTION (default: "")
If this option is set, it is used in the signature of the generated
jjtAccept() methods and the visit() methods. Note: this option will be
removed in a later version of JJTree. Don't use it if that bothers you.
JJTree state
JJTree keeps its state in a parser class field called jjtree. You can use
methods in this member to manipulate the node stack.
final class JJTreeState {
/* Call this to reinitialize the node stack. */
void reset();
/* Return the root node of the AST. */
Node rootNode();
/* Determine whether the current node was actually closed and
pushed */
boolean nodeCreated();
/* Return the number of nodes currently pushed on the node
stack in the current node scope. */
int arity();
/* Push a node on to the stack. */
void pushNode(Node n);
/* Return the node on the top of the stack, and remove it from the
stack. */
Node popNode();
/* Return the node currently on the top of the stack. */
Node peekNode();
}
Node Objects
/* All AST nodes must implement this interface. It provides basic
machinery for constructing the parent and child relationships
between nodes. */
public interface Node {
/** This method is called after the node has been made the current
node. It indicates that child nodes can now be added to it. */
public void jjtOpen();
/** This method is called after all the child nodes have been
added. */
public void jjtClose();
/** This pair of methods are used to inform the node of its
parent. */
public void jjtSetParent(Node n);
public Node jjtGetParent();
/** This method tells the node to add its argument to the node's
list of children. */
public void jjtAddChild(Node n, int i);
/** This method returns a child node. The children are numbered
from zero, left to right. */
public Node jjtGetChild(int i);
/** Return the number of children the node has. */
int jjtGetNumChildren();
}
The class SimpleNode implements the Node interface, and is automatically
generated by JJTree if it doesn't already exist. You can use this class as a
template or superclass for your node implementations, or you can modify it to
suit. SimpleNode additionally provides a rudimentary mechanism for recursively
dumping the node and its children. You might use this is in action like this:
{
((SimpleNode)jjtree.rootNode()).dump(">");
}
The String parameter to dump() is used as padding to indicate the tree
hierarchy.
Another utility method is generated if the VISITOR options is set:
{
public void childrenAccept(MyParserVisitor visitor);
}
This walks over the node's children in turn, asking them to accept the
visitor. This can be useful when implementing preorder and postorder
traversals.
More Documentation
Jocelyn Paine has contributed a very nice introduction to JJTree where he
describes how he has used it to develop an extension to HTML for interactive
web pages: http://users.ox.ac.uk/~popx/jjtree.html
Examples
JJTree 0.3pre3 is distributed with some simple examples containing a grammar
that parses arithmetic expressions. See the file
examples/JJTreeExamples/README for further details.
There is also an interpreter for a simple language that uses JJTree to build
the program representation. See the file examples/Interpreter/README for more
information.
A grammar for HTML 3.2 is also included in the distribution. See
examples/HTMLGrammars/RobsHTML/README to find out more.
Information about an example using the visitor support is in
examples/VTransformer/README.
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