JsonWriter.java

/*
 * Copyright (C) 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.gson.stream;

import static com.google.gson.stream.JsonScope.DANGLING_NAME;
import static com.google.gson.stream.JsonScope.EMPTY_ARRAY;
import static com.google.gson.stream.JsonScope.EMPTY_DOCUMENT;
import static com.google.gson.stream.JsonScope.EMPTY_OBJECT;
import static com.google.gson.stream.JsonScope.NONEMPTY_ARRAY;
import static com.google.gson.stream.JsonScope.NONEMPTY_DOCUMENT;
import static com.google.gson.stream.JsonScope.NONEMPTY_OBJECT;

import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.gson.FormattingStyle;
import com.google.gson.Strictness;
import java.io.Closeable;
import java.io.Flushable;
import java.io.IOException;
import java.io.Writer;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.Arrays;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.regex.Pattern;

/**
 * Writes a JSON (<a href="https://www.ietf.org/rfc/rfc8259.txt">RFC 8259</a>)
 * encoded value to a stream, one token at a time. The stream includes both
 * literal values (strings, numbers, booleans and nulls) as well as the begin
 * and end delimiters of objects and arrays.
 *
 * <h2>Encoding JSON</h2>
 * To encode your data as JSON, create a new {@code JsonWriter}. Call methods
 * on the writer as you walk the structure's contents, nesting arrays and objects
 * as necessary:
 * <ul>
 *   <li>To write <strong>arrays</strong>, first call {@link #beginArray()}.
 *       Write each of the array's elements with the appropriate {@link #value}
 *       methods or by nesting other arrays and objects. Finally close the array
 *       using {@link #endArray()}.
 *   <li>To write <strong>objects</strong>, first call {@link #beginObject()}.
 *       Write each of the object's properties by alternating calls to
 *       {@link #name} with the property's value. Write property values with the
 *       appropriate {@link #value} method or by nesting other objects or arrays.
 *       Finally close the object using {@link #endObject()}.
 * </ul>
 *
 * <h2>Example</h2>
 * Suppose we'd like to encode a stream of messages such as the following: <pre> {@code
 * [
 *   {
 *     "id": 912345678901,
 *     "text": "How do I stream JSON in Java?",
 *     "geo": null,
 *     "user": {
 *       "name": "json_newb",
 *       "followers_count": 41
 *      }
 *   },
 *   {
 *     "id": 912345678902,
 *     "text": "@json_newb just use JsonWriter!",
 *     "geo": [50.454722, -104.606667],
 *     "user": {
 *       "name": "jesse",
 *       "followers_count": 2
 *     }
 *   }
 * ]}</pre>
 * This code encodes the above structure: <pre>   {@code
 *   public void writeJsonStream(OutputStream out, List<Message> messages) throws IOException {
 *     JsonWriter writer = new JsonWriter(new OutputStreamWriter(out, "UTF-8"));
 *     writer.setIndent("    ");
 *     writeMessagesArray(writer, messages);
 *     writer.close();
 *   }
 *
 *   public void writeMessagesArray(JsonWriter writer, List<Message> messages) throws IOException {
 *     writer.beginArray();
 *     for (Message message : messages) {
 *       writeMessage(writer, message);
 *     }
 *     writer.endArray();
 *   }
 *
 *   public void writeMessage(JsonWriter writer, Message message) throws IOException {
 *     writer.beginObject();
 *     writer.name("id").value(message.getId());
 *     writer.name("text").value(message.getText());
 *     if (message.getGeo() != null) {
 *       writer.name("geo");
 *       writeDoublesArray(writer, message.getGeo());
 *     } else {
 *       writer.name("geo").nullValue();
 *     }
 *     writer.name("user");
 *     writeUser(writer, message.getUser());
 *     writer.endObject();
 *   }
 *
 *   public void writeUser(JsonWriter writer, User user) throws IOException {
 *     writer.beginObject();
 *     writer.name("name").value(user.getName());
 *     writer.name("followers_count").value(user.getFollowersCount());
 *     writer.endObject();
 *   }
 *
 *   public void writeDoublesArray(JsonWriter writer, List<Double> doubles) throws IOException {
 *     writer.beginArray();
 *     for (Double value : doubles) {
 *       writer.value(value);
 *     }
 *     writer.endArray();
 *   }}</pre>
 *
 * <p>Each {@code JsonWriter} may be used to write a single JSON stream.
 * Instances of this class are not thread safe. Calls that would result in a
 * malformed JSON string will fail with an {@link IllegalStateException}.
 *
 * @author Jesse Wilson
 * @since 1.6
 */
public class JsonWriter implements Closeable, Flushable {

  // Syntax as defined by https://datatracker.ietf.org/doc/html/rfc8259#section-6
  private static final Pattern VALID_JSON_NUMBER_PATTERN = Pattern.compile("-?(?:0|[1-9][0-9]*)(?:\\.[0-9]+)?(?:[eE][-+]?[0-9]+)?");

  /*
   * From RFC 8259, "All Unicode characters may be placed within the
   * quotation marks except for the characters that must be escaped:
   * quotation mark, reverse solidus, and the control characters
   * (U+0000 through U+001F)."
   *
   * We also escape '\u2028' and '\u2029', which JavaScript interprets as
   * newline characters. This prevents eval() from failing with a syntax
   * error. http://code.google.com/p/google-gson/issues/detail?id=341
   */
  private static final String[] REPLACEMENT_CHARS;
  private static final String[] HTML_SAFE_REPLACEMENT_CHARS;
  static {
    REPLACEMENT_CHARS = new String[128];
    for (int i = 0; i <= 0x1f; i++) {
      REPLACEMENT_CHARS[i] = String.format("\\u%04x", i);
    }
    REPLACEMENT_CHARS['"'] = "\\\"";
    REPLACEMENT_CHARS['\\'] = "\\\\";
    REPLACEMENT_CHARS['\t'] = "\\t";
    REPLACEMENT_CHARS['\b'] = "\\b";
    REPLACEMENT_CHARS['\n'] = "\\n";
    REPLACEMENT_CHARS['\r'] = "\\r";
    REPLACEMENT_CHARS['\f'] = "\\f";
    HTML_SAFE_REPLACEMENT_CHARS = REPLACEMENT_CHARS.clone();
    HTML_SAFE_REPLACEMENT_CHARS['<'] = "\\u003c";
    HTML_SAFE_REPLACEMENT_CHARS['>'] = "\\u003e";
    HTML_SAFE_REPLACEMENT_CHARS['&'] = "\\u0026";
    HTML_SAFE_REPLACEMENT_CHARS['='] = "\\u003d";
    HTML_SAFE_REPLACEMENT_CHARS['\''] = "\\u0027";
  }

  /** The JSON output destination */
  private final Writer out;

  private int[] stack = new int[32];
  private int stackSize = 0;
  {
    push(EMPTY_DOCUMENT);
  }

  private FormattingStyle formattingStyle;
  // These fields cache data derived from the formatting style, to avoid having to
  // re-evaluate it every time something is written
  private String formattedColon;
  private String formattedComma;
  private boolean usesEmptyNewlineAndIndent;

  private Strictness strictness = Strictness.LEGACY_STRICT;

  private boolean htmlSafe;

  private String deferredName;

  private boolean serializeNulls = true;

  /**
   * Creates a new instance that writes a JSON-encoded stream to {@code out}.
   * For best performance, ensure {@link Writer} is buffered; wrapping in
   * {@link java.io.BufferedWriter BufferedWriter} if necessary.
   */
  public JsonWriter(Writer out) {
    this.out = Objects.requireNonNull(out, "out == null");
    setFormattingStyle(FormattingStyle.COMPACT);
  }

  /**
   * Sets the indentation string to be repeated for each level of indentation
   * in the encoded document. If {@code indent.isEmpty()} the encoded document
   * will be compact. Otherwise the encoded document will be more
   * human-readable.
   *
   * <p>This is a convenience method which overwrites any previously
   * {@linkplain #setFormattingStyle(FormattingStyle) set formatting style} with
   * either {@link FormattingStyle#COMPACT} if the given indent string is
   * empty, or {@link FormattingStyle#PRETTY} with the given indent if
   * not empty.
   *
   * @param indent a string containing only whitespace.
   */
  public final void setIndent(String indent) {
    if (indent.isEmpty()) {
      setFormattingStyle(FormattingStyle.COMPACT);
    } else {
      setFormattingStyle(FormattingStyle.PRETTY.withIndent(indent));
    }
  }

  /**
   * Sets the formatting style to be used in the encoded document.
   *
   * <p>Sets the various attributes to be used in the encoded document.
   * For example the indentation string to be repeated for each level of indentation.
   * Or the newline style, to accommodate various OS styles.</p>
   *
   * @param formattingStyle the formatting style to use, must not be {@code null}.
   * @since $next-version$
   */
  public final void setFormattingStyle(FormattingStyle formattingStyle) {
    this.formattingStyle = Objects.requireNonNull(formattingStyle);

    this.formattedComma = ",";
    if (this.formattingStyle.usesSpaceAfterSeparators()) {
      this.formattedColon = ": ";

      // Only add space if no newline is written
      if (this.formattingStyle.getNewline().isEmpty()) {
        this.formattedComma = ", ";
      }
    } else {
      this.formattedColon = ":";
    }

    this.usesEmptyNewlineAndIndent = this.formattingStyle.getNewline().isEmpty()
        && this.formattingStyle.getIndent().isEmpty();
  }

  /**
   * Returns the pretty printing style used by this writer.
   *
   * @return the {@code FormattingStyle} that will be used.
   * @since $next-version$
   */
  public final FormattingStyle getFormattingStyle() {
    return formattingStyle;
  }

  /**
   * Sets the strictness of this writer.
   *
   * <p>This method is deprecated. Please use {@link JsonWriter#setStrictness(Strictness)} instead.
   * {@code JsonWriter.setLenient(true)} should be replaced by {@code JsonWriter.setStrictness(Strictness.LENIENT)}
   * and {@code JsonWriter.setLenient(false)} should be replaced by {@code JsonWriter.setStrictness(Strictness.LEGACY_STRICT)}.
   *
   * @param lenient whether this writer should be lenient. If true, the strictness is set to {@link Strictness#LENIENT}.
   *                If false, the strictness is set to {@link Strictness#LEGACY_STRICT}.
   * @see #setStrictness(Strictness)
   */
  @Deprecated
  public final void setLenient(boolean lenient) {
    this.strictness = lenient ? Strictness.LENIENT : Strictness.LEGACY_STRICT;
  }

  /**
   * Returns true if the {@link Strictness} of this writer is equal to {@link Strictness#LENIENT}.
   *
   * @see JsonWriter#setStrictness(Strictness)
   */
  public boolean isLenient() {
    return strictness == Strictness.LENIENT;
  }

  /**
   * Configures how strict this writer is with regard to the syntax rules specified in <a
   * href="https://www.ietf.org/rfc/rfc8259.txt">RFC 8259</a>. By default, {@link Strictness#LEGACY_STRICT} is used.
   *
   * <dl>
   *     <dt>{@link Strictness#STRICT} &amp; {@link Strictness#LEGACY_STRICT}</dt>
   *     <dd>
   *         The behavior of these is currently identical. In these strictness modes, the writer only writes JSON
   *         in accordance with <a href="https://www.ietf.org/rfc/rfc8259.txt">RFC 8259</a>.
   *     </dd>
   *     <dt>{@link Strictness#LENIENT}</dt>
   *     <dd>
   *         This mode relaxes the behavior of the writer to allow the writing of {@link Double#isNaN() NaNs}
   *         and {@link Double#isInfinite() infinities}. It also allows writing multiple top level values.
   *     </dd>
   * </dl>
   *
   * @param strictness the new strictness of this writer. May not be {@code null}.
   * @since $next-version$
   */
  public final void setStrictness(Strictness strictness) {
    this.strictness = Objects.requireNonNull(strictness);
  }

  /**
   * Returns how strict this writer is.
   *
   * @see #setStrictness(Strictness)
   * @since $next-version$
   */
  public final Strictness getStrictness() {
    return strictness;
  }

  /**
   * Configures this writer to emit JSON that's safe for direct inclusion in HTML
   * and XML documents. This escapes the HTML characters {@code <}, {@code >},
   * {@code &} and {@code =} before writing them to the stream. Without this
   * setting, your XML/HTML encoder should replace these characters with the
   * corresponding escape sequences.
   */
  public final void setHtmlSafe(boolean htmlSafe) {
    this.htmlSafe = htmlSafe;
  }

  /**
   * Returns true if this writer writes JSON that's safe for inclusion in HTML
   * and XML documents.
   */
  public final boolean isHtmlSafe() {
    return htmlSafe;
  }

  /**
   * Sets whether object members are serialized when their value is null.
   * This has no impact on array elements. The default is true.
   */
  public final void setSerializeNulls(boolean serializeNulls) {
    this.serializeNulls = serializeNulls;
  }

  /**
   * Returns true if object members are serialized when their value is null.
   * This has no impact on array elements. The default is true.
   */
  public final boolean getSerializeNulls() {
    return serializeNulls;
  }

  /**
   * Begins encoding a new array. Each call to this method must be paired with
   * a call to {@link #endArray}.
   *
   * @return this writer.
   */
  @CanIgnoreReturnValue
  public JsonWriter beginArray() throws IOException {
    writeDeferredName();
    return open(EMPTY_ARRAY, '[');
  }

  /**
   * Ends encoding the current array.
   *
   * @return this writer.
   */
  @CanIgnoreReturnValue
  public JsonWriter endArray() throws IOException {
    return close(EMPTY_ARRAY, NONEMPTY_ARRAY, ']');
  }

  /**
   * Begins encoding a new object. Each call to this method must be paired
   * with a call to {@link #endObject}.
   *
   * @return this writer.
   */
  @CanIgnoreReturnValue
  public JsonWriter beginObject() throws IOException {
    writeDeferredName();
    return open(EMPTY_OBJECT, '{');
  }

  /**
   * Ends encoding the current object.
   *
   * @return this writer.
   */
  @CanIgnoreReturnValue
  public JsonWriter endObject() throws IOException {
    return close(EMPTY_OBJECT, NONEMPTY_OBJECT, '}');
  }

  /**
   * Enters a new scope by appending any necessary whitespace and the given
   * bracket.
   */
  @CanIgnoreReturnValue
  private JsonWriter open(int empty, char openBracket) throws IOException {
    beforeValue();
    push(empty);
    out.write(openBracket);
    return this;
  }

  /**
   * Closes the current scope by appending any necessary whitespace and the
   * given bracket.
   */
  @CanIgnoreReturnValue
  private JsonWriter close(int empty, int nonempty, char closeBracket)
      throws IOException {
    int context = peek();
    if (context != nonempty && context != empty) {
      throw new IllegalStateException("Nesting problem.");
    }
    if (deferredName != null) {
      throw new IllegalStateException("Dangling name: " + deferredName);
    }

    stackSize--;
    if (context == nonempty) {
      newline();
    }
    out.write(closeBracket);
    return this;
  }

  private void push(int newTop) {
    if (stackSize == stack.length) {
      stack = Arrays.copyOf(stack, stackSize * 2);
    }
    stack[stackSize++] = newTop;
  }

  /**
   * Returns the value on the top of the stack.
   */
  private int peek() {
    if (stackSize == 0) {
      throw new IllegalStateException("JsonWriter is closed.");
    }
    return stack[stackSize - 1];
  }

  /**
   * Replace the value on the top of the stack with the given value.
   */
  private void replaceTop(int topOfStack) {
    stack[stackSize - 1] = topOfStack;
  }

  /**
   * Encodes the property name.
   *
   * @param name the name of the forthcoming value. May not be {@code null}.
   * @return this writer.
   */
  @CanIgnoreReturnValue
  public JsonWriter name(String name) throws IOException {
    Objects.requireNonNull(name, "name == null");
    if (deferredName != null) {
      throw new IllegalStateException();
    }
    if (stackSize == 0) {
      throw new IllegalStateException("JsonWriter is closed.");
    }
    deferredName = name;
    return this;
  }

  private void writeDeferredName() throws IOException {
    if (deferredName != null) {
      beforeName();
      string(deferredName);
      deferredName = null;
    }
  }

  /**
   * Encodes {@code value}.
   *
   * @param value the literal string value, or null to encode a null literal.
   * @return this writer.
   */
  @CanIgnoreReturnValue
  public JsonWriter value(String value) throws IOException {
    if (value == null) {
      return nullValue();
    }
    writeDeferredName();
    beforeValue();
    string(value);
    return this;
  }

  /**
   * Writes {@code value} directly to the writer without quoting or
   * escaping. This might not be supported by all implementations, if
   * not supported an {@code UnsupportedOperationException} is thrown.
   *
   * @param value the literal string value, or null to encode a null literal.
   * @return this writer.
   * @throws UnsupportedOperationException if this writer does not support
   *    writing raw JSON values.
   * @since 2.4
   */
  @CanIgnoreReturnValue
  public JsonWriter jsonValue(String value) throws IOException {
    if (value == null) {
      return nullValue();
    }
    writeDeferredName();
    beforeValue();
    out.append(value);
    return this;
  }

  /**
   * Encodes {@code null}.
   *
   * @return this writer.
   */
  @CanIgnoreReturnValue
  public JsonWriter nullValue() throws IOException {
    if (deferredName != null) {
      if (serializeNulls) {
        writeDeferredName();
      } else {
        deferredName = null;
        return this; // skip the name and the value
      }
    }
    beforeValue();
    out.write("null");
    return this;
  }

  /**
   * Encodes {@code value}.
   *
   * @return this writer.
   */
  @CanIgnoreReturnValue
  public JsonWriter value(boolean value) throws IOException {
    writeDeferredName();
    beforeValue();
    out.write(value ? "true" : "false");
    return this;
  }

  /**
   * Encodes {@code value}.
   *
   * @return this writer.
   * @since 2.7
   */
  @CanIgnoreReturnValue
  public JsonWriter value(Boolean value) throws IOException {
    if (value == null) {
      return nullValue();
    }
    writeDeferredName();
    beforeValue();
    out.write(value ? "true" : "false");
    return this;
  }

  /**
   * Encodes {@code value}.
   *
   * @param value a finite value, or if {@link #setLenient(boolean) lenient},
   *     also {@link Float#isNaN() NaN} or {@link Float#isInfinite()
   *     infinity}.
   * @return this writer.
   * @throws IllegalArgumentException if the value is NaN or Infinity and this writer is not {@link
   *     #setLenient(boolean) lenient}.
   * @since 2.9.1
   */
  @CanIgnoreReturnValue
  public JsonWriter value(float value) throws IOException {
    writeDeferredName();
    if (strictness != Strictness.LENIENT && (Float.isNaN(value) || Float.isInfinite(value))) {
      throw new IllegalArgumentException("Numeric values must be finite, but was " + value);
    }
    beforeValue();
    out.append(Float.toString(value));
    return this;
  }

  /**
   * Encodes {@code value}.
   *
   * @param value a finite value, or if {@link #setLenient(boolean) lenient},
   *     also {@link Double#isNaN() NaN} or {@link Double#isInfinite() infinity}.
   * @return this writer.
   * @throws IllegalArgumentException if the value is NaN or Infinity and this writer is
   *     not {@link #setLenient(boolean) lenient}.
   */
  @CanIgnoreReturnValue
  public JsonWriter value(double value) throws IOException {
    writeDeferredName();
    if (strictness != Strictness.LENIENT && (Double.isNaN(value) || Double.isInfinite(value))) {
      throw new IllegalArgumentException("Numeric values must be finite, but was " + value);
    }
    beforeValue();
    out.append(Double.toString(value));
    return this;
  }

  /**
   * Encodes {@code value}.
   *
   * @return this writer.
   */
  @CanIgnoreReturnValue
  public JsonWriter value(long value) throws IOException {
    writeDeferredName();
    beforeValue();
    out.write(Long.toString(value));
    return this;
  }

  /**
   * Returns whether the {@code toString()} of {@code c} can be trusted to return
   * a valid JSON number.
   */
  private static boolean isTrustedNumberType(Class<? extends Number> c) {
    // Note: Don't consider LazilyParsedNumber trusted because it could contain
    // an arbitrary malformed string
    return c == Integer.class || c == Long.class || c == Double.class || c == Float.class || c == Byte.class || c == Short.class
        || c == BigDecimal.class || c == BigInteger.class || c == AtomicInteger.class || c == AtomicLong.class;
  }

  /**
   * Encodes {@code value}. The value is written by directly writing the {@link Number#toString()}
   * result to JSON. Implementations must make sure that the result represents a valid JSON number.
   *
   * @param value a finite value, or if {@link #setLenient(boolean) lenient},
   *     also {@link Double#isNaN() NaN} or {@link Double#isInfinite() infinity}.
   * @return this writer.
   * @throws IllegalArgumentException if the value is NaN or Infinity and this writer is
   *     not {@link #setLenient(boolean) lenient}; or if the {@code toString()} result is not a
   *     valid JSON number.
   */
  @CanIgnoreReturnValue
  public JsonWriter value(Number value) throws IOException {
    if (value == null) {
      return nullValue();
    }

    writeDeferredName();
    String string = value.toString();
    if (string.equals("-Infinity") || string.equals("Infinity") || string.equals("NaN")) {
      if (strictness != Strictness.LENIENT) {
        throw new IllegalArgumentException("Numeric values must be finite, but was " + string);
      }
    } else {
      Class<? extends Number> numberClass = value.getClass();
      // Validate that string is valid before writing it directly to JSON output
      if (!isTrustedNumberType(numberClass) && !VALID_JSON_NUMBER_PATTERN.matcher(string).matches()) {
        throw new IllegalArgumentException("String created by " + numberClass + " is not a valid JSON number: " + string);
      }
    }

    beforeValue();
    out.append(string);
    return this;
  }

  /**
   * Ensures all buffered data is written to the underlying {@link Writer}
   * and flushes that writer.
   */
  @Override public void flush() throws IOException {
    if (stackSize == 0) {
      throw new IllegalStateException("JsonWriter is closed.");
    }
    out.flush();
  }

  /**
   * Flushes and closes this writer and the underlying {@link Writer}.
   *
   * @throws IOException if the JSON document is incomplete.
   */
  @Override public void close() throws IOException {
    out.close();

    int size = stackSize;
    if (size > 1 || (size == 1 && stack[size - 1] != NONEMPTY_DOCUMENT)) {
      throw new IOException("Incomplete document");
    }
    stackSize = 0;
  }

  private void string(String value) throws IOException {
    String[] replacements = htmlSafe ? HTML_SAFE_REPLACEMENT_CHARS : REPLACEMENT_CHARS;
    out.write('\"');
    int last = 0;
    int length = value.length();
    for (int i = 0; i < length; i++) {
      char c = value.charAt(i);
      String replacement;
      if (c < 128) {
        replacement = replacements[c];
        if (replacement == null) {
          continue;
        }
      } else if (c == '\u2028') {
        replacement = "\\u2028";
      } else if (c == '\u2029') {
        replacement = "\\u2029";
      } else {
        continue;
      }
      if (last < i) {
        out.write(value, last, i - last);
      }
      out.write(replacement);
      last = i + 1;
    }
    if (last < length) {
      out.write(value, last, length - last);
    }
    out.write('\"');
  }

  private void newline() throws IOException {
    if (usesEmptyNewlineAndIndent) {
      return;
    }

    out.write(formattingStyle.getNewline());
    for (int i = 1, size = stackSize; i < size; i++) {
      out.write(formattingStyle.getIndent());
    }
  }

  /**
   * Inserts any necessary separators and whitespace before a name. Also
   * adjusts the stack to expect the name's value.
   */
  private void beforeName() throws IOException {
    int context = peek();
    if (context == NONEMPTY_OBJECT) { // first in object
      out.write(formattedComma);
    } else if (context != EMPTY_OBJECT) { // not in an object!
      throw new IllegalStateException("Nesting problem.");
    }
    newline();
    replaceTop(DANGLING_NAME);
  }

  /**
   * Inserts any necessary separators and whitespace before a literal value,
   * inline array, or inline object. Also adjusts the stack to expect either a
   * closing bracket or another element.
   */
  @SuppressWarnings("fallthrough")
  private void beforeValue() throws IOException {
    switch (peek()) {
    case NONEMPTY_DOCUMENT:
      if (strictness != Strictness.LENIENT) {
        throw new IllegalStateException(
            "JSON must have only one top-level value.");
      }
      // fall-through
    case EMPTY_DOCUMENT: // first in document
      replaceTop(NONEMPTY_DOCUMENT);
      break;

    case EMPTY_ARRAY: // first in array
      replaceTop(NONEMPTY_ARRAY);
      newline();
      break;

    case NONEMPTY_ARRAY: // another in array
      out.append(formattedComma);
      newline();
      break;

    case DANGLING_NAME: // value for name
      out.append(formattedColon);
      replaceTop(NONEMPTY_OBJECT);
      break;

    default:
      throw new IllegalStateException("Nesting problem.");
    }
  }
}