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Spring Boot Test: druid WebStatFilter NullPointerException
测试 Controller 时报了如下错误，根据日志是由于 druid 的 WebStatFilter 中的 webAppStat 字段为 null 导致的，而这个字段是在 WebStatFilter.init() 方法中初始化的，也就是说 init() 方法在应用启动时没有被调用。
java
java.lang.NullPointerException
at com.alibaba.druid.support.http.WebStatFilter.doFilter(WebStatFilter.java:94)没被初始化的原因是由于：在 @SpringBootTest 注解的默认配置下（ webEnvironment 为 WebEnvironment.MOCK ），SpringApplication 指定 GenericWebApplicationContext 为应用的上下文，而它不会创建 servlet 容器，也不会初始化 Filter、 Servlet、Linstener [1]。
将 webEnvironment 指定为 WebEnvironment.RANDOM_PORT 或 WebEnvironment.DEFINED_PORT 即可避免这个错误：
java
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)说起来这个也有点类似于 TestRestTemplate bean 的初始化，只有当 webEnvironment.embedded 为 true 时才会被自动注入[2]。
附 1. 完整的错误日志
java
java.lang.NullPointerException
at com.alibaba.druid.support.http.WebStatFilter.doFilter(WebStatFilter.java:94)
at org.springframework.test.web.servlet.setup.PatternMappingFilterProxy.doFilter(PatternMappingFilterProxy.java:101)
at org.springframework.mock.web.MockFilterChain.doFilter(MockFilterChain.java:133)
at me.liujiajia.junit4.filter.UserInfoFilter.doFilter(UserInfoFilter.java:40)
at org.springframework.test.web.servlet.setup.PatternMappingFilterProxy.doFilter(PatternMappingFilterProxy.java:101)
at org.springframework.mock.web.MockFilterChain.doFilter(MockFilterChain.java:133)
at org.springframework.boot.actuate.web.trace.servlet.HttpTraceFilter.doFilterInternal(HttpTraceFilter.java:88)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
at org.springframework.mock.web.MockFilterChain.doFilter(MockFilterChain.java:133)
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
at org.springframework.mock.web.MockFilterChain.doFilter(MockFilterChain.java:133)
at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:92)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
at org.springframework.mock.web.MockFilterChain.doFilter(MockFilterChain.java:133)
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
at org.springframework.mock.web.MockFilterChain.doFilter(MockFilterChain.java:133)
at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.filterAndRecordMetrics(WebMvcMetricsFilter.java:114)
at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:104)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
at org.springframework.mock.web.MockFilterChain.doFilter(MockFilterChain.java:133)
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118)
at org.springframework.mock.web.MockFilterChain.doFilter(MockFilterChain.java:133)
at org.springframework.test.web.servlet.MockMvc.perform(MockMvc.java:182)
at me.liujiajia.junit4.controller.SomeControllerTest.page(SomeControllerTest.java:45)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:74)
at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:84)
at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
at com.intellij.rt.junit.IdeaTestRunner$Repeater$1.execute(IdeaTestRunner.java:38)
at com.intellij.rt.execution.junit.TestsRepeater.repeat(TestsRepeater.java:11)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:35)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)