Permit multiple top-level values in JsonWriter in lenient mode. Also fix some cases where we don't throw the right thing on a closed JsonWriter.

I'd prefer to not support multiple top-level values, but we support it in JsonReader and it's easier to be consistent. Kevin Hayen's patch pointed me in the right direction here, but I needed to do more work to cover some of the edge cases.

Fixes issue 397.

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

@@ -314,7 +314,11 @@ public class JsonWriter implements Closeable {
    * Returns the value on the top of the stack.
    */
   private JsonScope peek() {
-    return stack.get(stack.size() - 1);
+    int size = stack.size();
+    if (size == 0) {
+      throw new IllegalStateException("JsonWriter is closed.");
+    }
+    return stack.get(size - 1);
   }
 
   /**
@@ -337,6 +341,9 @@ public class JsonWriter implements Closeable {
     if (deferredName != null) {
       throw new IllegalStateException();
     }
+    if (stack.isEmpty()) {
+      throw new IllegalStateException("JsonWriter is closed.");
+    }
     deferredName = name;
     return this;
   }
@@ -453,6 +460,9 @@ public class JsonWriter implements Closeable {
    * and flushes that writer.
    */
   public void flush() throws IOException {
+    if (stack.isEmpty()) {
+      throw new IllegalStateException("JsonWriter is closed.");
+    }
     out.flush();
   }
 
@@ -464,9 +474,11 @@ public class JsonWriter implements Closeable {
   public void close() throws IOException {
     out.close();
 
-    if (peek() != JsonScope.NONEMPTY_DOCUMENT) {
+    int size = stack.size();
+    if (size > 1 || size == 1 && stack.get(size - 1) != JsonScope.NONEMPTY_DOCUMENT) {
       throw new IOException("Incomplete document");
     }
+    stack.clear();
   }
 
   private void string(String value) throws IOException {
@@ -574,8 +586,15 @@ public class JsonWriter implements Closeable {
    * @param root true if the value is a new array or object, the two values
    *     permitted as top-level elements.
    */
+  @SuppressWarnings("fallthrough")
   private void beforeValue(boolean root) throws IOException {
     switch (peek()) {
+    case NONEMPTY_DOCUMENT:
+      if (!lenient) {
+        throw new IllegalStateException(
+            "JSON must have only one top-level value.");
+      }
+      // fall-through
     case EMPTY_DOCUMENT: // first in document
       if (!lenient && !root) {
         throw new IllegalStateException(
@@ -599,10 +618,6 @@ public class JsonWriter implements Closeable {
       replaceTop(JsonScope.NONEMPTY_OBJECT);
       break;
 
-    case NONEMPTY_DOCUMENT:
-        throw new IllegalStateException(
-            "JSON must have only one top-level value.");
-
     default:
       throw new IllegalStateException("Nesting problem: " + stack);
     }

gson/src/test/java/com/google/gson/stream/JsonWriterTest.java

@@ -16,12 +16,11 @@
 
 package com.google.gson.stream;
 
-import junit.framework.TestCase;
-
 import java.io.IOException;
 import java.io.StringWriter;
 import java.math.BigDecimal;
 import java.math.BigInteger;
+import junit.framework.TestCase;
 
 public final class JsonWriterTest extends TestCase {
 
@@ -464,4 +463,104 @@ public final class JsonWriterTest extends TestCase {
         + "]";
     assertEquals(expected, stringWriter.toString());
   }
+
+  public void testLenientWriterPermitsMultipleTopLevelValues() throws IOException {
+    StringWriter stringWriter = new StringWriter();
+    JsonWriter writer = new JsonWriter(stringWriter);
+    writer.setLenient(true);
+    writer.beginArray();
+    writer.endArray();
+    writer.beginArray();
+    writer.endArray();
+    writer.close();
+    assertEquals("[][]", stringWriter.toString());
+  }
+
+  public void testStrictWriterDoesNotPermitMultipleTopLevelValues() throws IOException {
+    StringWriter stringWriter = new StringWriter();
+    JsonWriter writer = new JsonWriter(stringWriter);
+    writer.beginArray();
+    writer.endArray();
+    try {
+      writer.beginArray();
+      fail();
+    } catch (IllegalStateException expected) {
+    }
+  }
+
+  public void testClosedWriterThrowsOnStructure() throws IOException {
+    StringWriter stringWriter = new StringWriter();
+    JsonWriter writer = new JsonWriter(stringWriter);
+    writer.beginArray();
+    writer.endArray();
+    writer.close();
+    try {
+      writer.beginArray();
+      fail();
+    } catch (IllegalStateException expected) {
+    }
+    try {
+      writer.endArray();
+      fail();
+    } catch (IllegalStateException expected) {
+    }
+    try {
+      writer.beginObject();
+      fail();
+    } catch (IllegalStateException expected) {
+    }
+    try {
+      writer.endObject();
+      fail();
+    } catch (IllegalStateException expected) {
+    }
+  }
+
+  public void testClosedWriterThrowsOnName() throws IOException {
+    StringWriter stringWriter = new StringWriter();
+    JsonWriter writer = new JsonWriter(stringWriter);
+    writer.beginArray();
+    writer.endArray();
+    writer.close();
+    try {
+      writer.name("a");
+      fail();
+    } catch (IllegalStateException expected) {
+    }
+  }
+
+  public void testClosedWriterThrowsOnValue() throws IOException {
+    StringWriter stringWriter = new StringWriter();
+    JsonWriter writer = new JsonWriter(stringWriter);
+    writer.beginArray();
+    writer.endArray();
+    writer.close();
+    try {
+      writer.value("a");
+      fail();
+    } catch (IllegalStateException expected) {
+    }
+  }
+
+  public void testClosedWriterThrowsOnFlush() throws IOException {
+    StringWriter stringWriter = new StringWriter();
+    JsonWriter writer = new JsonWriter(stringWriter);
+    writer.beginArray();
+    writer.endArray();
+    writer.close();
+    try {
+      writer.flush();
+      fail();
+    } catch (IllegalStateException expected) {
+    }
+  }
+
+  public void testWriterCloseIsIdempotent() throws IOException {
+    StringWriter stringWriter = new StringWriter();
+    JsonWriter writer = new JsonWriter(stringWriter);
+    writer.beginArray();
+    writer.endArray();
+    writer.close();
+    writer.close();
+  }
 }