e the columns empty to signal wildcard } else { // Read names until we see a ',' do { String columnName = tokens.consume(); if (tokens.canConsume("AS")) { String columnAlias = tokens.consume(); columns.add(new Column(columnName, columnAlias)); } else { columns.add(new Column(columnName, null)); } } while (tokens.canConsume(',')); } return columns; } protected Delete parseDelete( TokenStream tokens ) throws ParsingException { tokens.consume("DELETE", "FROM"); String tableName = tokens.consume(); tokens.consume("WHERE"); String lhs = tokens.consume(); tokens.consume('='); String rhs = tokens.consume(); return new Delete(tableName, new Criteria(lhs, rhs)); } } public abstract class Statement { ... } public class Query extends Statement { ... } public class Delete extends Statement { ... } public class Column { ... } This example shows an idiomatic way of writing a parser that is stateless and thread-safe. The
parse(...) method takes the input as a parameter, and returns the domain-specific representation that resulted from the parsing. All other methods are utility methods that simply encapsulate common logic or make the code more readable.
In the example, the parse(...) first creates a TokenStream object (using a Tokenizer implementation that is not shown), and then loops as long as there are more tokens to read. As it loops, if the next token is "SELECT", the parser calls the parseSelect(...) method which immediately consumes a "SELECT" token, the names of the columns separated by commas (or a '*' if there all columns are to be selected), a "FROM" token, and the name of the table being queried. The parseSelect(...) method returns a Select object, which then added to the list of statements in the parse(...) method. The parser handles the "DELETE" statements in a similar manner.
Case sensitivity
Very often grammars to not require the case of keywords to match. This can make parsing a challenge, because all combinations of case need to be used. The TokenStream framework provides a very simple solution that requires no more effort than providing a boolean parameter to the constructor.
When a false value is provided for the the caseSensitive parameter, the TokenStream performs all matching operations as if each token's value were in uppercase only. This means that the arguments supplied to the match(...), canConsume(...), and consume(...) methods should be upper-cased. Note that the actual value of each token remains the actual case as it appears in the input.
Of course, when the TokenStream is created with a true value for the caseSensitive parameter, the matching is performed using the actual value as it appears in the input content
Whitespace
Many grammars are independent of lines breaks or whitespace, allowing a lot of flexibility when writing the content. The TokenStream framework makes it very easy to ignore line breaks and whitespace. To do so, the Tokenizer implementation must simply not include the line break character sequences and whitespace in the token ranges. Since none of the tokens contain whitespace, the parser never has to deal with them.
Of course, many parsers will require that some whitespace be included. For example, whitespace within a quoted string may be needed by the parser. In this case, the Tokenizer should simply include the whitespace characters in the tokens.
Writing a Tokenizer
Each parser will likely have its own {@link Tokenizer} implementation that contains the parser-specific logic about how tobreak the content into token objects. Generally, the easiest way to do this is to simply iterate through the character sequence passed into the {@link Tokenizer#tokenize(CharacterStream,Tokens) tokenize(...)} method, and use a switch statement to decidewhat to do.
Here is the code for a very basic Tokenizer implementation that ignores whitespace, line breaks and Java-style (multi-line and end-of-line) comments, while constructing single tokens for each quoted string.
public class BasicTokenizer implements Tokenizer { public void tokenize( CharacterStream input, Tokens tokens ) throws ParsingException { while (input.hasNext()) { char c = input.next(); switch (c) { case ' ': case '\t': case '\n': case '\r': // Just skip these whitespace characters ... break; case '-': case '(': case ')': case '{': case '}': case '*': case ',': case ';': case '+': case '%': case '?': case '$': case '[': case ']': case '!': case '<': case '>': case '|': case '=': case ':': tokens.addToken(input.index(), input.index() + 1, SYMBOL); break; case '.': tokens.addToken(input.index(), input.index() + 1, DECIMAL); break; case '\"': case '\"': int startIndex = input.index(); Position startingPosition = input.position(); boolean foundClosingQuote = false; while (input.hasNext()) { c = input.next(); if (c == '\\' && input.isNext('"')) { c = input.next(); // consume the ' character since it is escaped } else if (c == '"') { foundClosingQuote = true; break; } } if (!foundClosingQuote) { throw new ParsingException(startingPosition, "No matching closing double quote found"); } int endIndex = input.index() + 1; // beyond last character read tokens.addToken(startIndex, endIndex, DOUBLE_QUOTED_STRING); break; case '\'': startIndex = input.index(); startingPosition = input.position(); foundClosingQuote = false; while (input.hasNext()) { c = input.next(); if (c == '\\' && input.isNext('\'')) { c = input.next(); // consume the ' character since it is escaped } else if (c == '\'') { foundClosingQuote = true; break; } } if (!foundClosingQuote) { throw new ParsingException(startingPosition, "No matching closing single quote found"); } endIndex = input.index() + 1; // beyond last character read tokens.addToken(startIndex, endIndex, SINGLE_QUOTED_STRING); break; case '/': startIndex = input.index(); if (input.isNext('/')) { // End-of-line comment ... boolean foundLineTerminator = false; while (input.hasNext()) { c = input.next(); if (c == '\n' || c == '\r') { foundLineTerminator = true; break; } } endIndex = input.index(); // the token won't include the '\n' or '\r' character(s) if (!foundLineTerminator) ++endIndex; // must point beyond last char if (c == '\r' && input.isNext('\n')) input.next(); if (useComments) { tokens.addToken(startIndex, endIndex, COMMENT); } } else if (input.isNext('*')) { // Multi-line comment ... while (input.hasNext() && !input.isNext('*', '/')) { c = input.next(); } if (input.hasNext()) input.next(); // consume the '*' if (input.hasNext()) input.next(); // consume the '/' if (useComments) { endIndex = input.index() + 1; // the token will include the '/' and '*' characters tokens.addToken(startIndex, endIndex, COMMENT); } } else { // just a regular slash ... tokens.addToken(startIndex, startIndex + 1, SYMBOL); } break; default: startIndex = input.index(); // Read until another whitespace/symbol/decimal/slash is found while (input.hasNext() && !(input.isNextWhitespace() || input.isNextAnyOf("/.-(){}*,;+%?$[]!<>|=:"))) { c = input.next(); } endIndex = input.index() + 1; // beyond last character that was included tokens.addToken(startIndex, endIndex, WORD); } } } } Tokenizers with exactly this behavior can actually be created using the {@link #basicTokenizer(boolean)} method. So while this verybasic implementation is not meant to be used in all situations, it may be useful in some situations.