AST Traversal and Bytecode Generation in Custom Language Parser
Lets generate parser for this imaginary code :
.<cfst x = 10> <cfif x eq 5> <cfoutput>x</cfoutput> </cfif>! Let’s outline how the node classes can be generated and provide examples of what they might look like. These classes represent nodes in the Abstract Syntax Tree (AST) and are generated by JJTree based on the grammar rules defined in your parser.
Here’s how you might define the grammar file (CustomLanguageParser.jjt) with placeholders for the node classes:
options {
NODE_DEFAULT_VOID = true;
}
PARSER_BEGIN(CustomLanguageParser)
import java.io.*;
public class CustomLanguageParser {
public static void main(String[] args) throws ParseException, IOException {
if (args.length != 1) {
System.err.println("Usage: java CustomLanguageParser <input_file>");
System.exit(1);
}
FileInputStream inputStream = new FileInputStream(args[0]);
CustomLanguageParser parser = new CustomLanguageParser(inputStream);
Start();
}
}
PARSER_END(CustomLanguageParser)
TOKEN :
{
< CFIF : "<cfif" >
| < CFST : "<cfst" >
| < CFOUTPUT : "<cfoutput" >
| < TAG_CLOSE : "</" >
| < TAG_END : ">" >
| < IDENTIFIER : (["a"-"z","A"-"Z"])+ >
| < INTEGER : (["0"-"9"])+ >
| < STRING : "\"" (~["\""])* "\"" >
| < EOL : "\n" | "\r" | "\r\n" >
}
void Start() :
{}
{
(Statement())*
}
void Statement() :
{}
{
(ConditionalStatement() | AssignmentStatement() | OutputStatement()) < EOL >
}
void ConditionalStatement() :
{}
{
CFIF Identifier() Comparison() Value() < TAG_END > Statement() TAG_CLOSE "cfif" #IfStatement
}
void AssignmentStatement() :
{}
{
CFST Identifier() "=" Value() TAG_END #AssignmentStatement
}
void OutputStatement() :
{}
{
CFOUTPUT Identifier() TAG_CLOSE "cfoutput" #OutputStatement
}
void Identifier() :
{}
{
< IDENTIFIER >
}
void Value() :
{}
{
< INTEGER > | < STRING >
}
void Comparison() :
{}
{
"eq" | "neq" | "lt" | "gt" | "lte" | "gte"
}
The #IfStatement, #AssignmentStatement, and #OutputStatement directives in the grammar rules instruct JJTree to generate node classes named IfStatement, AssignmentStatement, and OutputStatement, respectively, for these rules.
Here’s an example of what the generated AssignmentStatement node class might look like:
public class AssignmentStatement extends SimpleNode {
private String identifier;
private String value;
public AssignmentStatement(int id) {
super(id);
}
public void setIdentifier(String identifier) {
this.identifier = identifier;
}
public void setValue(String value) {
this.value = value;
}
// Other methods for accessing identifier and value
}
public class IfStatement extends SimpleNode {
private String identifier;
private String comparison;
private String value;
public IfStatement(int id) {
super(id);
}
public void setIdentifier(String identifier) {
this.identifier = identifier;
}
public void setComparison(String comparison) {
this.comparison = comparison;
}
public void setValue(String value) {
this.value = value;
}
// Other methods for accessing identifier, comparison, and value
}
public class OutputStatement extends SimpleNode {
private String identifier;
public OutputStatement(int id) {
super(id);
}
public void setIdentifier(String identifier) {
this.identifier = identifier;
}
// Other methods for accessing identifier
}
These are just examples, and the actual structure of the node classes will depend on your grammar rules. Each node class should extend SimpleNode, which is provided by JJTree. You’ll need to examine the generated classes to understand their structure and use them appropriately in your code for AST traversal and bytecode generation.
Bytecode Generation:
We’ll traverse the AST and generate bytecode for each node in a separate class.
import org.apache.bcel.generic.*;
import java.io.*;
import java.util.*;
public class Main {
public static void main(String[] args) throws IOException, ParseException {
if (args.length != 1) {
System.err.println("Usage: java Main <input_file>");
System.exit(1);
}
// Parse the input file
FileInputStream inputStream = new FileInputStream(args[0]);
CustomLanguageParser parser = new CustomLanguageParser(inputStream);
SimpleNode ast = parser.Start();
// Generate bytecode
generateBytecode(ast);
}
public static void generateBytecode(SimpleNode ast) throws IOException {
// Create a new JavaClass
JavaClass parserClass = new JavaClass(
"CustomLanguageParser",
"java.lang.Object",
"<generated>",
Const.ACC_PUBLIC | Const.ACC_SUPER,
null
);
// Create a new MethodGen
MethodGen mainMethod = new MethodGen(
Const.ACC_PUBLIC | Const.ACC_STATIC,
Type.VOID,
new Type[]{},
new String[]{},
"main",
"CustomLanguageParser",
new InstructionList(),
new ConstantPoolGen()
);
// Traverse AST and generate bytecode for each node
InstructionList il = mainMethod.getInstructionList();
ConstantPoolGen cp = mainMethod.getConstantPoolGen();
generateBytecodeForNode(ast, mainMethod, il, cp);
// Finish main method
il.append(new RETURN());
mainMethod.setMaxStack();
mainMethod.setMaxLocals();
parserClass.addMethod(mainMethod.getMethod());
// Write class to file
parserClass.dump(new FileOutputStream("CustomLanguageParser.class"));
}
private static void generateBytecodeForNode(SimpleNode node, MethodGen method, InstructionList il, ConstantPoolGen cp) {
switch (node.getId()) {
case CustomLanguageParserTreeConstants.JJTASSIGNMENTSTATEMENT:
AssignmentStatement assignmentNode = (AssignmentStatement) node;
// Generate bytecode for assignment node
generateBytecodeForAssignment(assignmentNode, method, il, cp);
break;
case CustomLanguageParserTreeConstants.JJTOUTPUTSTATEMENT:
OutputStatement outputNode = (OutputStatement) node;
// Generate bytecode for output node
generateBytecodeForOutput(outputNode, method, il, cp);
break;
case CustomLanguageParserTreeConstants.JJTIFSTATEMENT:
IfStatement ifNode = (IfStatement) node;
// Generate bytecode for if node
generateBytecodeForIf(ifNode, method, il, cp);
break;
// Add cases for other node types as needed
default:
// Ignore other node types
break;
}
} private static void generateBytecodeForAssignment(AssignmentStatement node, MethodGen method, InstructionList il, ConstantPoolGen cp) {
// Generate bytecode for assignment node
// For example:
// Load value onto stack
il.append(new PUSH(cp, Integer.parseInt(node.getValue())));
// Store value into local variable
il.append(new ASTORE(method.getLocalVariable(node.getIdentifier(), Type.INT)));
}
private static void generateBytecodeForOutput(OutputStatement node, MethodGen method, InstructionList il, ConstantPoolGen cp) {
// Generate bytecode for output node
// For example:
// Load value onto stack
il.append(new PUSH(cp, Integer.parseInt(node.getIdentifier())));
// Get PrintStream object
il.append(new GETSTATIC(cp.addFieldref("java.lang.System", "out", "Ljava/io/PrintStream;")));
// Invoke println method
il.append(new INVOKEVIRTUAL(cp.addMethodref("java.io.PrintStream", "println", "(I)V")));
}
private static void generateBytecodeForIf(IfStatement node, MethodGen method, InstructionList il, ConstantPoolGen cp) {
// Generate bytecode for if node
// For example:
// Load value onto stack
il.append(new PUSH(cp, Integer.parseInt(node.getValue())));
// Compare value with 0
il.append(new IFNE(null)); // Branch to true case if value is not equal to 0
InstructionHandle falseCase = il.append(new NOP()); // Placeholder for false case
// Generate bytecode for statements inside if block
generateBytecodeForNode((SimpleNode) node.jjtGetChild(0), method, il, cp);
InstructionHandle trueCase = il.append(new NOP()); // Placeholder for true case
// Set branch target for false case
il.insert(falseCase, new GOTO(trueCase));
// Set branch target for true case
il.insert(trueCase, new NOP());
}
}
