java - 从列表中删除第二项会导致 REST API 崩溃

Question

我正在尝试向 REST API 添加一项功能，按年份过滤数据，这样只有对象才会显示与我请求的年份相同的对象。

我用来测试此功能的路径是

  http://localhost:8080/Assignment2C/beers?year=2019

出现 500 错误

在 Beers_Service.java 中，第二次迭代列表时失败

public List<Beers> getAllBeersByYear(int year) {
EntityManager em = DBUtil.getEMF().createEntityManager();

List<Beers> list = null;

try {
    list = em.createNamedQuery("Beers.findAll", Beers.class)
            .getResultList();
    if (list == null || list.isEmpty()) {
        list = null;
    }

} finally {
    em.close();
}
Calendar cal = Calendar.getInstance();
for(Beers beer: list){
    cal.setTime(beer.getLastMod());
    if(cal.get(Calendar.YEAR)!=year){

        list.remove(beer);
    }
}
return list;

数据库中的所有日期都是 2019 年，除了第一个对象是 2018 年

使用调试器时，它会迭代 for 循环，因为数据库中的第一个条目是 2018，所以它可以很好地删除它，删除对象后，它会返回到 for 循环的顶部，并在这一行崩溃.

 for(Beers beer: list){

当在 Debug模式下打破 for 循环后，它会进入此源代码中

                   beforeExecute(wt, task);
                    try {
                        task.run();
                        afterExecute(task, null);
                    } catch (Throwable ex) {
                        afterExecute(task, ex);
                        throw ex;
                    }

在线上

afterExecute(task, null);

我得到的堆栈跟踪是

<h1>HTTP Status 500 – Internal Server Error</h1>
    <hr class="line" />
    <p><b>Type</b> Exception Report</p>
    <p><b>Message</b> Request processing failed; nested exception is java.util.ConcurrentModificationException</p>
    <p><b>Description</b> The server encountered an unexpected condition that prevented it from fulfilling the request.
    </p>
    <p><b>Exception</b></p>
    <pre>org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.util.ConcurrentModificationException
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1014)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
</pre>
    <p><b>Root Cause</b></p>
    <pre>java.util.ConcurrentModificationException
    java.base&#47;java.util.Vector$Itr.checkForComodification(Vector.java:1320)
    java.base&#47;java.util.Vector$Itr.next(Vector.java:1276)
    main.Beers_Service.getAllBeersByYear(Beers_Service.java:53)
    main.Beers_Controller.GetAllBreweries(Beers_Controller.java:72)
    java.base&#47;jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    java.base&#47;jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    java.base&#47;jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    java.base&#47;java.lang.reflect.Method.invoke(Method.java:567)
    org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190)
    org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138)
    org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:105)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:879)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:793)
    org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

Answer 1

在迭代集合时不应修改集合: https://docs.oracle.com/javase/7/docs/api/java/util/ConcurrentModificationException.html

“例如，如果线程在使用快速失败迭代器迭代集合时直接修改集合，则迭代器将抛出此异常。”