Remove DateTypeAdapter to avoid code duplication (#2546)

The same functionality exists in `DefaultDateTypeAdapter` (with some slight
overhead), and there is currently code duplication between these classes.

gson/src/main/java/com/google/gson/Gson.java

@@ -25,7 +25,7 @@
 import com.google.gson.internal.Streams;
 import com.google.gson.internal.bind.ArrayTypeAdapter;
 import com.google.gson.internal.bind.CollectionTypeAdapterFactory;
-import com.google.gson.internal.bind.DateTypeAdapter;
+import com.google.gson.internal.bind.DefaultDateTypeAdapter;
 import com.google.gson.internal.bind.JsonAdapterAnnotationTypeAdapterFactory;
 import com.google.gson.internal.bind.JsonTreeReader;
 import com.google.gson.internal.bind.JsonTreeWriter;
@@ -369,7 +369,7 @@ public Gson() {
     factories.add(TypeAdapters.LOCALE_FACTORY);
     factories.add(TypeAdapters.INET_ADDRESS_FACTORY);
     factories.add(TypeAdapters.BIT_SET_FACTORY);
-    factories.add(DateTypeAdapter.FACTORY);
+    factories.add(DefaultDateTypeAdapter.DEFAULT_STYLE_FACTORY);
     factories.add(TypeAdapters.CALENDAR_FACTORY);
 
     if (SqlTypesSupport.SUPPORTS_SQL_TYPES) {

gson/src/main/java/com/google/gson/internal/bind/DefaultDateTypeAdapter.java

@@ -16,12 +16,14 @@
 
 package com.google.gson.internal.bind;
 
+import com.google.gson.Gson;
 import com.google.gson.JsonSyntaxException;
 import com.google.gson.TypeAdapter;
 import com.google.gson.TypeAdapterFactory;
 import com.google.gson.internal.JavaVersion;
 import com.google.gson.internal.PreJava9DateFormatProvider;
 import com.google.gson.internal.bind.util.ISO8601Utils;
+import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonToken;
 import com.google.gson.stream.JsonWriter;
@@ -35,17 +37,48 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Objects;
+import java.util.TimeZone;
 
 /**
  * This type adapter supports subclasses of date by defining a {@link
  * DefaultDateTypeAdapter.DateType} and then using its {@code createAdapterFactory} methods.
  *
+ * <p><b>Important:</b> Instances of this class (or rather the {@link SimpleDateFormat} they use)
+ * capture the current default {@link Locale} and {@link TimeZone} when they are created. Therefore
+ * avoid storing factories obtained from {@link DateType} in {@code static} fields, since they only
+ * create a single adapter instance and its behavior would then depend on when Gson classes are
+ * loaded first, and which default {@code Locale} and {@code TimeZone} was used at that point.
+ *
  * @author Inderjeet Singh
  * @author Joel Leitch
  */
 public final class DefaultDateTypeAdapter<T extends Date> extends TypeAdapter<T> {
   private static final String SIMPLE_NAME = "DefaultDateTypeAdapter";
 
+  /** Factory for {@link Date} adapters which use {@link DateFormat#DEFAULT} as style. */
+  public static final TypeAdapterFactory DEFAULT_STYLE_FACTORY =
+      // Because SimpleDateFormat captures the default TimeZone when it was created, let the factory
+      // always create new DefaultDateTypeAdapter instances (which are then cached by the Gson
+      // instances) instead of having a single static DefaultDateTypeAdapter instance
+      // Otherwise the behavior would depend on when an application first loads Gson classes and
+      // which default TimeZone is set at that point, which would be quite brittle
+      new TypeAdapterFactory() {
+        @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
+        @Override
+        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+          return typeToken.getRawType() == Date.class
+              ? (TypeAdapter<T>)
+                  new DefaultDateTypeAdapter<>(
+                      DateType.DATE, DateFormat.DEFAULT, DateFormat.DEFAULT)
+              : null;
+        }
+
+        @Override
+        public String toString() {
+          return "DefaultDateTypeAdapter#DEFAULT_STYLE_FACTORY";
+        }
+      };
+
   public abstract static class DateType<T extends Date> {
     public static final DateType<Date> DATE =
         new DateType<Date>(Date.class) {
@@ -123,8 +156,6 @@ private DefaultDateTypeAdapter(DateType<T> dateType, int dateStyle, int timeStyl
     }
   }
 
-  // These methods need to be synchronized since JDK DateFormat classes are not thread-safe
-  // See issue 162
   @Override
   public void write(JsonWriter out, Date value) throws IOException {
     if (value == null) {
@@ -134,6 +165,7 @@ public void write(JsonWriter out, Date value) throws IOException {
 
     DateFormat dateFormat = dateFormats.get(0);
     String dateFormatAsString;
+    // Needs to be synchronized since JDK DateFormat classes are not thread-safe
     synchronized (dateFormats) {
       dateFormatAsString = dateFormat.format(value);
     }
@@ -152,6 +184,7 @@ public T read(JsonReader in) throws IOException {
 
   private Date deserializeToDate(JsonReader in) throws IOException {
     String s = in.nextString();
+    // Needs to be synchronized since JDK DateFormat classes are not thread-safe
     synchronized (dateFormats) {
       for (DateFormat dateFormat : dateFormats) {
         try {