Groovy example source code file (SimpleTemplateEngine.java)

This example Groovy source code file (SimpleTemplateEngine.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Groovy tags/keywords

binding, binding, groovyshell, io, ioexception, ioexception, printwriter, simpletemplate, simpletemplateengine, simpletemplateengine, string, stringwriter, stringwriter, template, util, writable

The Groovy SimpleTemplateEngine.java source code

/*
 * Copyright 2003-2009 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package groovy.text;

import groovy.lang.*;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.Reader;
import java.io.StringWriter;
import java.io.Writer;
import java.util.Map;

import org.codehaus.groovy.control.CompilationFailedException;
import org.codehaus.groovy.runtime.InvokerHelper;

/**
 * Processes template source files substituting variables and expressions into
 * placeholders in a template source text to produce the desired output.
 * </P>
 * The template engine uses JSP style <% %> script and <%= %> expression syntax
 * or GString style expressions. The variable '<code>out' is bound to the writer that the template
 * is being written to.
 * </p>
 * Frequently, the template source will be in a file but here is a simple
 * example providing the template as a string:
 * <pre>
 * def binding = [
 *     firstname : "Grace",
 *     lastname  : "Hopper",
 *     accepted  : true,
 *     title     : 'Groovy for COBOL programmers'
 * ]
 * def engine = new groovy.text.SimpleTemplateEngine()
 * def text = '''\
 * Dear <%= firstname %> $lastname,
 *
 * We <% if (accepted) print 'are pleased' else print 'regret' %> \
 * to inform you that your paper entitled
 * '$title' was ${ accepted ? 'accepted' : 'rejected' }.
 *
 * The conference committee.
 * '''
 * def template = engine.createTemplate(text).make(binding)
 * println template.toString()
 * </pre>
 * This example uses a mix of the JSP style and GString style placeholders
 * but you can typically use just one style if you wish. Running this
 * example will produce this output:
 * <pre>
 * Dear Grace Hopper,
 *
 * We are pleased to inform you that your paper entitled
 * 'Groovy for COBOL programmers' was accepted.
 *
 * The conference committee.
 * </pre>
 * The template engine can also be used as the engine for {@link groovy.servlet.TemplateServlet} by placing the
 * following in your <code>web.xml file (plus a corresponding servlet-mapping element):
 * <pre>
 * <servlet>
 *   <servlet-name>SimpleTemplate</servlet-name>
 *   <servlet-class>groovy.servlet.TemplateServlet</servlet-class>
 *   <init-param>
 *     <param-name>template.engine</param-name>
 *     <param-value>groovy.text.SimpleTemplateEngine</param-value>
 *   </init-param>
 * </servlet>
 * </pre>
 * In this case, your template source file should be HTML with the appropriate embedded placeholders.
 *
 * @author sam
 * @author Christian Stein
 * @author Paul King
 * @author Alex Tkachman
 */
public class SimpleTemplateEngine extends TemplateEngine {
    private boolean verbose;
    private static int counter = 1;

    private GroovyShell groovyShell;

    public SimpleTemplateEngine() {
        this(GroovyShell.class.getClassLoader());
    }

    public SimpleTemplateEngine(boolean verbose) {
        this(GroovyShell.class.getClassLoader());
        setVerbose(verbose);
    }

    public SimpleTemplateEngine(ClassLoader parentLoader) {
        this(new GroovyShell(parentLoader));
    }

    public SimpleTemplateEngine(GroovyShell groovyShell) {
        this.groovyShell = groovyShell;
    }

    public Template createTemplate(Reader reader) throws CompilationFailedException, IOException {
        SimpleTemplate template = new SimpleTemplate();
        String script = template.parse(reader);
        if (verbose) {
            System.out.println("\n-- script source --");
            System.out.print(script);
            System.out.println("\n-- script end --\n");
        }
        try {
            template.script = groovyShell.parse(script, "SimpleTemplateScript" + counter++ + ".groovy");
        } catch (Exception e) {
            throw new GroovyRuntimeException("Failed to parse template script (your template may contain an error or be trying to use expressions not currently supported): " + e.getMessage());
        }
        return template;
    }

    /**
     * @param verbose true if you want the engine to display the template source file for debugging purposes
     */
    public void setVerbose(boolean verbose) {
        this.verbose = verbose;
    }

    public boolean isVerbose() {
        return verbose;
    }

    private static class SimpleTemplate implements Template {

        protected Script script;

        public Writable make() {
            return make(null);
        }

        public Writable make(final Map map) {
            return new Writable() {
                /**
                 * Write the template document with the set binding applied to the writer.
                 *
                 * @see groovy.lang.Writable#writeTo(java.io.Writer)
                 */
                public Writer writeTo(Writer writer) {
                    Binding binding;
                    if (map == null)
                        binding = new Binding();
                    else
                        binding = new Binding(map);
                    Script scriptObject = InvokerHelper.createScript(script.getClass(), binding);
                    PrintWriter pw = new PrintWriter(writer);
                    scriptObject.setProperty("out", pw);
                    scriptObject.run();
                    pw.flush();
                    return writer;
                }

                /**
                 * Convert the template and binding into a result String.
                 *
                 * @see java.lang.Object#toString()
                 */
                public String toString() {
                    StringWriter sw = new StringWriter();
                    writeTo(sw);
                    return sw.toString();
                }
            };
        }

        /**
         * Parse the text document looking for <% or <%= and then call out to the appropriate handler, otherwise copy the text directly
         * into the script while escaping quotes.
         *
         * @param reader a reader for the template text
         * @return the parsed text
         * @throws IOException if something goes wrong
         */
        protected String parse(Reader reader) throws IOException {
            if (!reader.markSupported()) {
                reader = new BufferedReader(reader);
            }
            StringWriter sw = new StringWriter();
            startScript(sw);
            int c;
            while ((c = reader.read()) != -1) {
                if (c == '<') {
                    reader.mark(1);
                    c = reader.read();
                    if (c != '%') {
                        sw.write('<');
                        reader.reset();
                    } else {
                        reader.mark(1);
                        c = reader.read();
                        if (c == '=') {
                            groovyExpression(reader, sw);
                        } else {
                            reader.reset();
                            groovySection(reader, sw);
                        }
                    }
                    continue; // at least '<' is consumed ... read next chars.
                }
                if (c == '$') {
                    reader.mark(1);
                    c = reader.read();
                    if (c != '{') {
                        sw.write('$');
                        reader.reset();
                    } else {
                        reader.mark(1);
                        sw.write("${");
                        processGSstring(reader, sw);
                    }
                    continue; // at least '$' is consumed ... read next chars.
                }
                if (c == '\"') {
                    sw.write('\\');
                }
                /*
                 * Handle raw new line characters.
                 */
                if (c == '\n' || c == '\r') {
                    if (c == '\r') { // on Windows, "\r\n" is a new line.
                        reader.mark(1);
                        c = reader.read();
                        if (c != '\n') {
                            reader.reset();
                        }
                    }
                    sw.write("\n");
                    continue;
                }
                sw.write(c);
            }
            endScript(sw);
            return sw.toString();
        }

        private void startScript(StringWriter sw) {
            sw.write("/* Generated by SimpleTemplateEngine */\n");
            sw.write("out.print(\"\"\"");
        }

        private void endScript(StringWriter sw) {
            sw.write("\"\"\");\n");
        }

        private void processGSstring(Reader reader, StringWriter sw) throws IOException {
            int c;
            while ((c = reader.read()) != -1) {
                if (c != '\n' && c != '\r') {
                    sw.write(c);
                }
                if (c == '}') {
                    break;
                }
            }
        }

        /**
         * Closes the currently open write and writes out the following text as a GString expression until it reaches an end %>.
         *
         * @param reader a reader for the template text
         * @param sw     a StringWriter to write expression content
         * @throws IOException if something goes wrong
         */
        private void groovyExpression(Reader reader, StringWriter sw) throws IOException {
            sw.write("${");
            int c;
            while ((c = reader.read()) != -1) {
                if (c == '%') {
                    c = reader.read();
                    if (c != '>') {
                        sw.write('%');
                    } else {
                        break;
                    }
                }
                if (c != '\n' && c != '\r') {
                    sw.write(c);
                }
            }
            sw.write("}");
        }

        /**
         * Closes the currently open write and writes the following text as normal Groovy script code until it reaches an end %>.
         *
         * @param reader a reader for the template text
         * @param sw     a StringWriter to write expression content
         * @throws IOException if something goes wrong
         */
        private void groovySection(Reader reader, StringWriter sw) throws IOException {
            sw.write("\"\"\");");
            int c;
            while ((c = reader.read()) != -1) {
                if (c == '%') {
                    c = reader.read();
                    if (c != '>') {
                        sw.write('%');
                    } else {
                        break;
                    }
                }
                /* Don't eat EOL chars in sections - as they are valid instruction separators.
                 * See http://jira.codehaus.org/browse/GROOVY-980
                 */
                // if (c != '\n' && c != '\r') {
                sw.write(c);
                //}
            }
            sw.write(";\nout.print(\"\"\"");
        }

    }
}