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/*
* Copyright (C) 2018 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;
/**
* Caches {@link Method} objects for public methods accessed through references. The first time
* we evaluate {@code $var.property} or {@code $var.method(...)} for a {@code $var} of a given
* class and for a given property or method signature, we'll store the resultant {@link Method}
* object. Every subsequent time we'll reuse that {@link Method}. The method lookup is quite slow
* so caching is useful. The main downside is that we may potentially hold on to {@link Method}
* objects that will never be used with this {@link Template} again. But in practice templates
* tend to be used repeatedly with the same classes.
*/
private final MethodFinder methodFinder = new MethodFinder();
/**
* 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>{@code
* 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 {@code 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.
* @return a {@code Reader} for the resource.
* @throws IOException if the resource cannot be opened.
*/
Reader openResource(String resourceName) throws IOException;
}
/**
* Parses a VTL template from the given {@code Reader}. The template cannot reference other
* templates (for example with {@code #parse}). For that, use
* {@link #parseFrom(String, ResourceOpener)}.
*
* @param reader a Reader that will supply the text of the template. It will be closed on return
* from this method.
* @return an object representing the parsed template.
* @throws IOException if there is an exception reading from {@code reader}, or if the template
* references another template via {@code #parse}.
*/
public static Template parseFrom(Reader reader) throws IOException {
ResourceOpener resourceOpener = resourceName -> {
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 the initial resource and resources referenced by
* {@code #parse} directives in the template.
* @return an object representing the parsed template.
* @throws IOException if there is an exception opening or reading from any resource.
*/
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, methodFinder);
return String.valueOf(root.evaluate(evaluationContext));
}
}
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