Debugging Spring MVC @Service Bean Injection Issues with JMap and JHat

Spring MVC is widely used in Meituan-Dianping teams. Annotations such as @Override, @Deprecated, @Controller, @Service, @Autowired, and custom annotations are scattered throughout the codebase. While annotations simplify configuration, misuse can cause subtle bugs.

Event Cause

A new HTTP interface was wrapped and deployed after passing local unit tests. During testing a NullPointerException occurred. The stack trace showed a ClientProtocolException caused by a missing target host, which was traced back to the URL property not being injected when the class was annotated with @Service.

ERROR [qtp384587033-86] 2015-12-21 16:29:00.905 com.meituan.trip.mobile.hermes.common.utils.HttpClientUtils.doRequest(HttpClientUtils.java:359) HttpClientUtils.doRequest invoke get error, url:nullmt/api/test/v1/query?id=123456
org.apache.http.client.ClientProtocolException
    at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:186) ~[httpclient-4.3.5.jar:4.3.5]
Caused by: org.apache.http.ProtocolException: Target host is not specified

Problem Localization

Keeping the @Service annotation and restarting the application revealed that the bean definition for queryPartnerImpl was being overridden.

INFO [main] 2015-12-21 16:28:47.078 org.springframework.beans.factory.support.DefaultListableBeanFactory.registerBeanDefinition Overriding bean definition for bean 'queryPartnerImpl': replacing [Generic bean: class [com.meituan.trip.mobile.hermes.sal.meilv.impl.QueryPartnerImpl]; scope=singleton; ...] with [Generic bean: class [com.meituan.trip.mobile.hermes.sal.meilv.impl.QueryPartnerImpl]; scope=; ...]

Spring replaces a bean when the same name is defined twice in the same WebApplicationContext. The log indicated that the XML‑defined bean from sal/service-outer.xml replaced the annotated one, but the application still failed.

Investigation with JMap

Using JMap to count live instances of QueryPartnerImpl showed two objects, contradicting the expectation of a single singleton.

$ jmap -histo:live 20881 | grep QueryPartnerImpl
1354:              2              80  com.meituan.trip.mobile.hermes.sal.meilv.impl.QueryPartnerImpl

A heap dump was created for deeper analysis.

$ jmap -dump:format=b,file=/tmp/heap.bin 20881
Dumping heap to /private/tmp/dump.data ...
Heap dump file created

The dump was examined with JHat (HTTP server on port 7000) and MAT to locate the two instances and their reference graphs.

$ jhat /tmp/heap.bin
...........................................................................
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.

Instance details:

QueryPartnerImpl@0x6c41b6f80 (64 bytes)
clientId : trip_trade
clientSecret : 6ee952489a93b51b1ffcadd040ca562e
url : http://test.url.meituan.com/
...

QueryPartnerImpl@0x7aeafac20 (64 bytes)
clientId : <null>
clientSecret : <null>
url : <null>
...

The first instance had all properties injected correctly; the second had null values, which caused the NPE.

Root Cause Analysis

Reference tracing showed that the faulty instance was held by DispatcherServlet 's child WebApplicationContext, while the correct one was held by the root WebApplicationContext created by ContextLoaderListener. The duplication originated from both the root and child contexts loading the same package with annotation scanning, while sal/service-out.xml also defined the bean via XML, leading to replacement.

Solutions

Remove the @Service annotation (not ideal if other annotations are needed).

Isolate scanning by configuring use-default-filters and using include-filter/exclude-filter to limit which annotations are picked up.

Adjust spring-servlet.xml and applicationContext.xml accordingly (example screenshot omitted).

Key take‑aways:

Annotations are powerful but must be used consistently; property injection should also be annotation‑driven.

When enabling component scanning, carefully define the scan scope and filters.

Unit tests may pass because they load only a single configuration, hiding duplicate‑bean issues.

Follow framework best‑practice guidelines to avoid mysterious runtime problems.

Further reading includes the Spring Web MVC documentation and the Meituan-Dianping technical blog.