src/main/java/com/google/escapevelocity/Template.java

/*
 * Copyright (C) 2015 Google, Inc.
 *
 * 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 com.google.escapevelocity;

import com.google.escapevelocity.EvaluationContext.PlainEvaluationContext;
import java.io.IOException;
import java.io.Reader;
import java.util.Map;

/**
 * A template expressed in EscapeVelocity, a subset of the Velocity Template Language (VTL) from
 * Apache. The intent of this implementation is that if a template is accepted and successfully
 * produces output, that output will be identical to what Velocity would have produced for the same
 * template and input variables.
 *
 * @author emcmanus@google.com (Éamonn McManus)
 */
// TODO(emcmanus): spell out exactly what Velocity features are unsupported.
public class Template {
  private final Node root;

  /**
   * Used to resolve references to resources in the template, through {@code #parse} directives.
   *
   * <p>Here is an example that opens nested templates as resources relative to the calling class:
   *
   * <pre>
   *   ResourceOpener resourceOpener = resourceName -> {
   *     InputStream inputStream = getClass().getResource(resourceName);
   *     if (inputStream == null) {
   *       throw new IOException("Unknown resource: " + resourceName);
   *     }
   *     return new BufferedReader(InputStreamReader(inputStream, StandardCharsets.UTF_8));
   *   };
   * </pre>
   */
  @FunctionalInterface
  public interface ResourceOpener {

    /**
     * Returns a Reader that will be used to read the given resource, then closed.
     *
     * @param resourceName the name of the resource to be read. This will never be null.
     */
    Reader openResource(String resourceName) throws IOException;
  }

  /**
   * Parses a VTL template from the given {@code Reader}. The given Reader will be closed on
   * return from this method.
   */
  public static Template parseFrom(final Reader reader) throws IOException {
    ResourceOpener resourceOpener = new ResourceOpener() {
      @Override
      public Reader openResource(String resourceName) throws IOException {
        if (resourceName == null) {
          return reader;
        } else {
          throw new IOException("No ResourceOpener has been configured to read " + resourceName);
        }
      }
    };
    try {
      return parseFrom((String) null, resourceOpener);
    } finally {
      reader.close();
    }
  }

  /**
   * Parse a VTL template of the given name using the given {@code ResourceOpener}.
   *
   * @param resourceName name of the resource. May be null.
   * @param resourceOpener used to open included files for {@code #parse} directives in the
   *     template.
   */
  public static Template parseFrom(
      String resourceName, ResourceOpener resourceOpener) throws IOException {
    try (Reader reader = resourceOpener.openResource(resourceName)) {
      return new Parser(reader, resourceName, resourceOpener).parse();
    }
  }

  Template(Node root) {
    this.root = root;
  }

  /**
   * Evaluate the given template with the given initial set of variables.
   *
   * @param vars a map where the keys are variable names and the values are the corresponding
   *     variable values. For example, if {@code "x"} maps to 23, then {@code $x} in the template
   *     will expand to 23.
   *
   * @return the string result of evaluating the template.
   */
  public String evaluate(Map<String, ?> vars) {
    EvaluationContext evaluationContext = new PlainEvaluationContext(vars);
    return String.valueOf(root.evaluate(evaluationContext));
  }
}