fix AfterFilter NullPointer exception in #3443

src/main/java/com/alibaba/fastjson/serializer/AfterFilter.java

@@ -23,9 +23,9 @@ protected final void writeKeyValue(String key, Object value) {
         JSONSerializer serializer = serializerLocal.get();
         char seperator = seperatorLocal.get();
 
-        boolean ref = serializer.references.containsKey(value);
+        boolean ref = serializer.containsReference(value);
         serializer.writeKeyValue(seperator, key, value);
-        if (!ref) {
+        if (!ref && serializer.references != null) {
             serializer.references.remove(value);
         }
         if (seperator != ',') {

src/test/java/com/alibaba/json/bvt/issue_3300/Issue3443.java

@@ -0,0 +1,58 @@
+package com.alibaba.json.bvt.issue_3300;
+
+import com.alibaba.fastjson.serializer.*;
+import junit.framework.TestCase;
+
+public class Issue3443 extends TestCase {
+    public void testCustomJsonSerializerAndAfterFilter() throws Exception {
+        SerializeWriter serializeWriter = new SerializeWriter();
+        try {
+            JSONSerializer jsonSerializer = new JSONSerializer(serializeWriter, new SerializeConfig());
+
+            Parameter parameter = new Parameter();
+            parameter.setParameterDesc(new ParameterDesc("vipExpireDate", "VIP expire date."));
+
+            jsonSerializer.config(SerializerFeature.DisableCircularReferenceDetect, true);
+            jsonSerializer.getAfterFilters().add(new CustomFilter());
+            jsonSerializer.write(parameter);
+            assertEquals("{\"parameterDesc\":{\"ParameterDesc\":\"VIP expire date.\"}}", serializeWriter.toString());
+        } finally {
+            serializeWriter.close();
+        }
+    }
+
+    static class Parameter {
+        private ParameterDesc parameterDesc;
+
+        public ParameterDesc getParameterDesc() {
+            return parameterDesc;
+        }
+
+        public void setParameterDesc(ParameterDesc parameterType) {
+            this.parameterDesc = parameterType;
+        }
+    }
+
+    static class ParameterDesc {
+        private String parameterName;
+        private String parameterUsage;
+        // do some work...
+
+        public ParameterDesc(String parameterName, String parameterUsage) {
+            this.parameterName = parameterName;
+            this.parameterUsage = parameterUsage;
+        }
+
+
+    }
+
+    static class CustomFilter extends AfterFilter {
+
+        @Override
+        public void writeAfter(Object object) {
+            if (object instanceof ParameterDesc) {
+                writeKeyValue("ParameterDesc", "VIP expire date.");
+            }
+        }
+    }
+}