Make StringQuoter date parsing thread-safe

user/src/com/google/web/bindery/autobean/shared/impl/StringQuoter.java

@@ -30,12 +30,20 @@
  */
 public class StringQuoter {
   private static final String ISO8601_PATTERN = "yyyy-MM-dd'T'HH:mm:ss.SSSz";
-  private static final DateFormat ISO8601 = new SimpleDateFormat(ISO8601_PATTERN, Locale
-      .getDefault());
+  private static final ThreadLocal<DateFormat> ISO8601 = new ThreadLocal<DateFormat>() {
+    @Override
+    protected DateFormat initialValue() {
+      return new SimpleDateFormat(ISO8601_PATTERN, Locale.getDefault());
+    }
+  };
 
   private static final String RFC2822_PATTERN = "EEE, d MMM yyyy HH:mm:ss Z";
-  private static final DateFormat RFC2822 = new SimpleDateFormat(RFC2822_PATTERN, Locale
-      .getDefault());
+  private static final ThreadLocal<DateFormat> RFC2822 = new ThreadLocal<DateFormat>() {
+    @Override
+    protected DateFormat initialValue() {
+      return new SimpleDateFormat(RFC2822_PATTERN, Locale.getDefault());
+    }
+  };
 
   public static Splittable create(boolean value) {
     return JsonSplittable.create(String.valueOf(value));
@@ -85,11 +93,11 @@ public static Date tryParseDate(String date) {
       date = date.substring(0, date.length() - 1) + "+0000";
     }
     try {
-      return ISO8601.parse(date);
+      return ISO8601.get().parse(date);
     } catch (ParseException ignored) {
     }
     try {
-      return RFC2822.parse(date);
+      return RFC2822.get().parse(date);
     } catch (ParseException ignored) {
     }
     return null;

user/test/com/google/web/bindery/autobean/AutoBeanSuite.java

@@ -22,6 +22,7 @@
 import com.google.web.bindery.autobean.vm.AutoBeanCodexJreTest;
 import com.google.web.bindery.autobean.vm.AutoBeanJreTest;
 import com.google.web.bindery.autobean.vm.SplittableJreTest;
+import com.google.web.bindery.autobean.vm.StringQuoterJreTest;
 
 import junit.framework.Test;
 
@@ -38,6 +39,7 @@ public static Test suite() {
     suite.addTestSuite(AutoBeanTest.class);
     suite.addTestSuite(SplittableJreTest.class);
     suite.addTestSuite(SplittableTest.class);
+    suite.addTestSuite(StringQuoterJreTest.class);
     return suite;
   }
 }

user/test/com/google/web/bindery/autobean/vm/StringQuoterJreTest.java

@@ -0,0 +1,115 @@
+/*
+ * Copyright 2026 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.web.bindery.autobean.vm;
+
+import com.google.web.bindery.autobean.shared.impl.StringQuoter;
+
+import junit.framework.TestCase;
+
+import java.text.SimpleDateFormat;
+import java.util.Date;
+import java.util.Locale;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
+/**
+ * Tests for the server-side {@link StringQuoter#tryParseDate(String)}.
+ */
+public class StringQuoterJreTest extends TestCase {
+
+  private static final String ISO8601_PATTERN = "yyyy-MM-dd'T'HH:mm:ss.SSSz";
+  private static final String RFC2822_PATTERN = "EEE, d MMM yyyy HH:mm:ss Z";
+
+  public void testTryParseDateMillis() {
+    Date d = new Date(1234567890123L);
+    assertEquals(d, StringQuoter.tryParseDate(Long.toString(d.getTime())));
+  }
+
+  public void testTryParseDateIso8601() {
+    SimpleDateFormat fmt = new SimpleDateFormat(ISO8601_PATTERN, Locale.getDefault());
+    Date d = new Date(1234567890123L);
+    assertEquals(d, StringQuoter.tryParseDate(fmt.format(d)));
+  }
+
+  public void testTryParseDateZuluSuffix() throws Exception {
+    SimpleDateFormat fmt = new SimpleDateFormat(ISO8601_PATTERN, Locale.getDefault());
+    Date expected = fmt.parse("2024-01-15T10:30:00.000+0000");
+    assertEquals(expected, StringQuoter.tryParseDate("2024-01-15T10:30:00.000Z"));
+  }
+
+  public void testTryParseDateRfc2822() {
+    // RFC 2822 has second resolution.
+    SimpleDateFormat fmt = new SimpleDateFormat(RFC2822_PATTERN, Locale.getDefault());
+    Date d = new Date(1234567890000L);
+    assertEquals(d, StringQuoter.tryParseDate(fmt.format(d)));
+  }
+
+  public void testTryParseDateUnparseable() {
+    assertNull(StringQuoter.tryParseDate("not a date"));
+  }
+
+  /**
+   * SimpleDateFormat is not thread-safe; sharing a single instance across
+   * request threads on the server produced either ParseExceptions (returned as
+   * null) or silently wrong Dates. Hammer tryParseDate from many threads and
+   * make sure every call returns the expected value.
+   */
+  public void testTryParseDateConcurrent() throws Exception {
+    SimpleDateFormat fmt = new SimpleDateFormat(ISO8601_PATTERN, Locale.getDefault());
+    final Date expected = new Date(1234567890123L);
+    final String input = fmt.format(expected);
+
+    final int threads = 16;
+    final int perThread = 2000;
+    final CountDownLatch start = new CountDownLatch(1);
+    final AtomicInteger nulls = new AtomicInteger();
+    final AtomicInteger wrong = new AtomicInteger();
+    ExecutorService pool = Executors.newFixedThreadPool(threads);
+    try {
+      for (int i = 0; i < threads; i++) {
+        pool.submit(new Runnable() {
+          @Override
+          public void run() {
+            try {
+              start.await();
+            } catch (InterruptedException e) {
+              Thread.currentThread().interrupt();
+              return;
+            }
+            for (int j = 0; j < perThread; j++) {
+              Date r = StringQuoter.tryParseDate(input);
+              if (r == null) {
+                nulls.incrementAndGet();
+              } else if (!expected.equals(r)) {
+                wrong.incrementAndGet();
+              }
+            }
+          }
+        });
+      }
+      start.countDown();
+      pool.shutdown();
+      assertTrue("threads did not finish in time", pool.awaitTermination(60, TimeUnit.SECONDS));
+    } finally {
+      pool.shutdownNow();
+    }
+    assertEquals("parses returned null", 0, nulls.get());
+    assertEquals("parses returned wrong date", 0, wrong.get());
+  }
+}