AVRO-3825: [Java] Disallow invalid namespaces

lang/java/avro/src/main/java/org/apache/avro/Schema.java

@@ -717,9 +717,7 @@ public Name(String name, String space) {
         space = name.substring(0, lastDot); // get space from name
         this.name = validateName(name.substring(lastDot + 1));
       }
-      if ("".equals(space))
-        space = null;
-      this.space = space;
+      this.space = validateSpace(space);
       this.full = (this.space == null) ? this.name : this.space + "." + this.name;
     }
 
@@ -1645,6 +1643,17 @@ private static String validateName(String name) {
     return name;
   }
 
+  private static String validateSpace(String space) {
+    if ("".equals(space) || space == null) {
+      return null;
+    }
+
+    for (String part : space.split("\\.")) {
+      validateName(part);
+    }
+    return space;
+  }
+
   private static final ThreadLocal<Boolean> VALIDATE_DEFAULTS = ThreadLocalWithInitial.of(() -> true);
 
   private static JsonNode validateDefault(String fieldName, Schema schema, JsonNode defaultValue) {

lang/java/avro/src/test/java/org/apache/avro/TestSchema.java

@@ -460,4 +460,17 @@ void add_types() {
     assertNotNull(f1);
     assertEquals(schemaRecord1, f1.schema());
   }
+
+  @Test
+  void testInvalidNamespace() throws Exception {
+    File avscFile = Files.createTempFile("testInvalidNamespace", null).toFile();
+    String schema = "{\"type\":\"record\", \"namespace\": \"foo.123.bar\", \"name\":\"my_record\", \"fields\":[]}";
+    try (FileWriter writer = new FileWriter(avscFile)) {
+      writer.write(schema);
+      writer.flush();
+    }
+
+    Schema.Parser parser = new Schema.Parser();
+    assertThrows(SchemaParseException.class, () -> parser.parse(avscFile));
+  }
 }