Throw UnsupportedOperationException when JsonWriter.jsonValue is not supported (#1651)

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

@@ -152,6 +152,10 @@ public final class JsonTreeWriter extends JsonWriter {
     return this;
   }
 
+  @Override public JsonWriter jsonValue(String value) throws IOException {
+    throw new UnsupportedOperationException();
+  }
+
   @Override public JsonWriter nullValue() throws IOException {
     put(JsonNull.INSTANCE);
     return this;

gson/src/main/java/com/google/gson/stream/JsonWriter.java

@@ -426,10 +426,13 @@ public class JsonWriter implements Closeable, Flushable {
 
   /**
    * Writes {@code value} directly to the writer without quoting or
-   * escaping.
+   * 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.
    */
   public JsonWriter jsonValue(String value) throws IOException {
     if (value == null) {

gson/src/test/java/com/google/gson/internal/bind/JsonTreeWriterTest.java

@@ -233,4 +233,14 @@ public final class JsonTreeWriterTest extends TestCase {
     } catch (IllegalArgumentException expected) {
     }
   }
+
+  public void testJsonValue() throws IOException {
+    JsonTreeWriter writer = new JsonTreeWriter();
+    writer.beginArray();
+    try {
+      writer.jsonValue("test");
+      fail();
+    } catch (UnsupportedOperationException expected) {
+    }
+  }
 }