Optimize Java JsonFormat string printing path.

java/util/src/main/java/com/google/protobuf/util/JsonFormat.java

@@ -1423,41 +1423,78 @@ private void printSingleFieldValue(
       }
     }
 
+    private static final String[] replacementChars;
+
+    static {
+      replacementChars = new String[128];
+      for (int i = 0; i <= 0x1f; i++) {
+        replacementChars[i] = String.format("\\u%04x", i);
+      }
+      replacementChars['"'] = "\\\"";
+      replacementChars['\\'] = "\\\\";
+      replacementChars['\t'] = "\\t";
+      replacementChars['\b'] = "\\b";
+      replacementChars['\n'] = "\\n";
+      replacementChars['\r'] = "\\r";
+      replacementChars['\f'] = "\\f";
+
+      // These characters are fully legal in JSON, but are escaped to prevent XSS risks. Notably
+      // this topic is not like 'html escaping' where these would be replaced with something like
+      // `&lt;`. The escaped or not ways of writing it are verbatim 2 exactly equivalent
+      // ways to represent the same exact value in JSON: consumers should never do any manual
+      // unescape to round trip the intended value. This replacement is only be semantically
+      // observable if someone tries to handle it raw textually and not as JSON.
+      for (int i : "<>&='".toCharArray()) {
+        replacementChars[i] = String.format("\\u%04x", i);
+      }
+    }
+
     /**
-     * Prints a string value wrapped in double quotes, escaping any illegal or dangerous characters
-     * for JSON safety.
+     * If a character needs to be escaped, returns the replacement string. Otherwise, returns null.
      */
-    private void printStringEscapedAndQuoted(final CharSequence value) throws IOException {
-      // gson.toJson() is expensive: only use it if the string isn't entirely safe to print
-      // directly.
-      if (isJsonSafeString(value)) {
-        generator.print("\"");
-        generator.print(value);
-        generator.print("\"");
-      } else {
-        generator.print(gson.toJson(value.toString()));
+    private static String getReplacementOrNull(char c) {
+      if (c < 128) {
+        return replacementChars[c];
+      }
+
+      // \u2028 and \u2029 are paragraph whitespace characters that had mismatch between the
+      // JSON spec and the browser JavaScript object literal spec. It is extremely unlikely
+      // this topic matters now, but for parity with GSON we escape them.
+      if (c == '\u2028') {
+        return "\\u2028";
+      }
+      if (c == '\u2029') {
+        return "\\u2029";
       }
+
+      return null;
     }
 
-    private static boolean isJsonSafeString(CharSequence value) {
+    /**
+     * Prints a string value wrapped in double quotes, escaping any illegal or dangerous characters
+     * for JSON safety.
+     */
+    private void printStringEscapedAndQuoted(final CharSequence value) throws IOException {
+      generator.print("\"");
       int len = value.length();
+      int last = 0;
       for (int i = 0; i < len; i++) {
         char c = value.charAt(i);
-        // Bare characters, fully disallowed in JSON strings and which must be escaped.
-        if (c < 0x20 || c == '"' || c == '\\') {
-          return false;
-        }
-        // HTML-sensitive characters. These are allowed in JSON, but escaped to mitigate
-        // XSS risks when the JSON is rendered in HTML.
-        if (c == '<' || c == '>' || c == '&' || c == '=' || c == '\'') {
-          return false;
+        String replacement = getReplacementOrNull(c);
+
+        // Keeps scanning to only call print() once on long runs that don't need escaping.
+        if (replacement == null) {
+          continue;
         }
-        // Non-ASCII characters are mostly safe, but we'll leave that to gson to decide.
-        if (c >= 127) {
-          return false;
+        if (last < i) {
+          generator.print(value.subSequence(last, i));
         }
+        generator.print(replacement);
+        last = i + 1;
       }
-      return true;
+
+      generator.print(value.subSequence(last, len));
+      generator.print("\"");
     }
   }
 

java/util/src/test/java/com/google/protobuf/util/JsonFormatTest.java

@@ -13,6 +13,7 @@
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
+import com.google.gson.Gson;
 import com.google.gson.JsonSyntaxException;
 import com.google.protobuf.Any;
 import com.google.protobuf.BoolValue;
@@ -1950,6 +1951,38 @@ public void testStringEscapingWithControlCharacters() throws Exception {
     assertThat(builder.getOptionalString()).isEqualTo(complexString);
   }
 
+  @Test
+  public void testStringEscapingAgainstGsonParity_allLowChars() throws Exception {
+    StringBuilder sb = new StringBuilder();
+    for (int i = 0; i <= 256; i++) {
+      sb.append((char) i);
+    }
+    String allChars = sb.toString();
+    TestAllTypes message = TestAllTypes.newBuilder().setOptionalString(allChars).build();
+    String gsonEscaped = new Gson().toJson(allChars);
+
+    String expectedJson = "{\n  \"optionalString\": " + gsonEscaped + "\n}";
+    assertThat(toJsonString(message)).isEqualTo(expectedJson);
+
+    TestAllTypes.Builder builder = TestAllTypes.newBuilder();
+    JsonFormat.parser().merge(toJsonString(message), builder);
+    assertThat(builder.getOptionalString()).isEqualTo(allChars);
+  }
+
+  @Test
+  public void testStringEscapingAgainstGsonParity_danglingSurrogate() throws Exception {
+    String danglingSurrogate = "foo \uD800 bar";
+    TestAllTypes message = TestAllTypes.newBuilder().setOptionalString(danglingSurrogate).build();
+    String gsonEscaped = new Gson().toJson(danglingSurrogate);
+
+    String expectedJson = "{\n  \"optionalString\": " + gsonEscaped + "\n}";
+    assertThat(toJsonString(message)).isEqualTo(expectedJson);
+
+    TestAllTypes.Builder builder = TestAllTypes.newBuilder();
+    JsonFormat.parser().merge(toJsonString(message), builder);
+    assertThat(builder.getOptionalString()).isEqualTo(danglingSurrogate);
+  }
+
   @Test
   public void testDefaultValueOptionsProto3() throws Exception {
     TestAllTypes message = TestAllTypes.getDefaultInstance();