user/src/com/google/gwt/safehtml/shared/SafeHtmlBuilder.java

/*
 * Copyright 2010 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.gwt.safehtml.shared;

/**
 * A builder that facilitates the building up of XSS-safe HTML from text
 * snippets. It is used essentially like a {@link StringBuilder}; unlike a
 * {@link StringBuilder}, it automatically HTML-escapes appended input where
 * necessary.
 *
 * <p>
 * In addition, it supports methods that allow strings with HTML markup to be
 * appended without escaping: One can append other {@link SafeHtml} objects, and
 * one can append constant strings. The method that appends constant strings
 * ({@link #appendHtmlConstant(String)}) requires a convention of use to be
 * adhered to in order for this class to adhere to the contract required by
 * {@link SafeHtml}: The argument expression must be fully determined and known
 * to be safe at compile time, and the value of the argument must not contain
 * incomplete HTML tags. See {@link #appendHtmlConstant(String)} for details.
 *
 * <p>
 * The accumulated XSS-safe HTML can be obtained in the form of a
 * {@link SafeHtml} via the {@link #toSafeHtml()} method.
 *
 * <p>
 * This class is not thread-safe.
 */
public final class SafeHtmlBuilder {

  private final StringBuilder sb = new StringBuilder();

  /**
   * Constructs an empty SafeHtmlBuilder.
   */
  public SafeHtmlBuilder() {
  }

  /*
   * Boolean and numeric types converted to String are always HTML safe -- no
   * escaping necessary.
   */

  /**
   * Appends the string representation of a boolean.
   *
   * @param b the boolean whose string representation to append
   * @return a reference to this object
   */
  public SafeHtmlBuilder append(boolean b) {
    sb.append(b);
    return this;
  }

  /**
   * Appends the string representation of a number.
   *
   * @param num the number whose string representation to append
   * @return a reference to this object
   */
  public SafeHtmlBuilder append(byte num) {
    sb.append(num);
    return this;
  }

  /**
   * Appends the string representation of a char.
   *
   * @param c the character whose string representation to append
   * @return a reference to this object
   * @see SafeHtmlUtils#htmlEscape(char)
   */
  public SafeHtmlBuilder append(char c) {
    sb.append(SafeHtmlUtils.htmlEscape(c));
    return this;
  }

  /**
   * Appends the string representation of a number.
   *
   * @param num the number whose string representation to append
   * @return a reference to this object
   */
  public SafeHtmlBuilder append(double num) {
    sb.append(num);
    return this;
  }

  /**
   * Appends the string representation of a number.
   *
   * @param num the number whose string representation to append
   * @return a reference to this object
   */
  public SafeHtmlBuilder append(float num) {
    sb.append(num);
    return this;
  }

  /**
   * Appends the string representation of a number.
   *
   * @param num the number whose string representation to append
   * @return a reference to this object
   */
  public SafeHtmlBuilder append(int num) {
    sb.append(num);
    return this;
  }

  /**
   * Appends the string representation of a number.
   *
   * @param num the number whose string representation to append
   * @return a reference to this object
   */
  public SafeHtmlBuilder append(long num) {
    sb.append(num);
    return this;
  }

  /**
   * Appends the contents of another {@link SafeHtml} object, without applying
   * HTML-escaping to it.
   *
   * @param html the {@link SafeHtml} to append
   * @return a reference to this object
   */
  public SafeHtmlBuilder append(SafeHtml html) {
    sb.append(html.asString());
    return this;
  }

  /**
   * Appends a string after HTML-escaping it.
   *
   * @param text the string to append
   * @return a reference to this object
   * @see SafeHtmlUtils#htmlEscape(String)
   */
  public SafeHtmlBuilder appendEscaped(String text) {
    sb.append(SafeHtmlUtils.htmlEscape(text));
    return this;
  }

  /**
   * Appends a string consisting of several newline-separated lines after
   * HTML-escaping it. Newlines in the original string are converted to {@code
   * <br>} tags.
   *
   * @param text the string to append
   * @return a reference to this object
   * @see SafeHtmlUtils#htmlEscape(String)
   */
  public SafeHtmlBuilder appendEscapedLines(String text) {
    sb.append(SafeHtmlUtils.htmlEscape(text).replaceAll("\n", "<br>"));
    return this;
  }

  /**
   * Appends a compile-time-constant string, which will <em>not</em> be escaped.
   *
   * <p>
   * <b>Important</b>: For this class to be able to honor its contract as
   * required by {@link SafeHtml}, all uses of this method must satisfy the
   * following constraints:
   *
   * <ol>
   *
   * <li>The argument expression must be fully determined at compile time.
   *
   * <li>The value of the argument must end in "inner HTML" context and not
   * contain incomplete HTML tags. I.e., the following is not a correct use of
   * this method, because the {@code <a>} tag is incomplete:
   *
   * <pre class="code">
   * {@code shb.appendConstantHtml("<a href='").append(url)}</pre>
   *
   * </ol>
   *
   * <p>
   * The first constraint provides a sufficient condition that the appended
   * string (and any HTML markup contained in it) originates from a trusted
   * source. The second constraint ensures the composability of {@link SafeHtml}
   * values.
   *
   * <p>
   * When executing client-side in Development Mode, or server side with
   * assertions enabled, the argument is HTML-parsed and validated to satisfy
   * the second constraint (the server-side check can also be enabled
   * programmatically, see
   * {@link SafeHtmlHostedModeUtils#maybeCheckCompleteHtml(String)} for
   * details). For performance reasons, this check is not performed in prod mode
   * on the client, and with assertions disabled on the server.
   *
   * @param html the HTML snippet to be appended
   * @return a reference to this object
   * @throws IllegalArgumentException if not running in prod mode and {@code
   *           html} violates the second constraint
   */
  public SafeHtmlBuilder appendHtmlConstant(String html) {
    SafeHtmlHostedModeUtils.maybeCheckCompleteHtml(html);
    sb.append(html);
    return this;
  }

  /**
   * Returns the safe HTML accumulated in the builder as a {@link SafeHtml}.
   *
   * @return a SafeHtml instance
   */
  public SafeHtml toSafeHtml() {
    return new SafeHtmlString(sb.toString());
  }
}