java.lang.RuntimeException : Unable to instantiate @Form class. 没有无参数构造函数

Question

我正在编写一个POJO，如下所示。

public class EndpointParams implements Serializable {
    private static final long serialVersionUID = -5269761907708414499L;

    @QueryParam("pageNum") @DefaultValue("1") private int pageNum;
    @QueryParam("pageSize") @DefaultValue("25") private int pageSize;

    public EndpointParams (int pageNum, int pageSize) {
        this.pageNum = pageNum;
        this.pageSize = pageSize;
    }

    public int getPageNum() {
        return pageNum;
    }

    public void setPageNum(int pageNum) {
        this.pageNum = pageNum;
    }

    public int getPageSize() {
        return pageSize;
    }

    public void setPageSize(int pageSize) {
        this.pageSize = pageSize;
    }

    @Override
    public boolean equals(Object obj) {
        // logic here
    }

    @Override
    public int hashCode() {
        // logic here
    }
} 

当我在 JBOSS 上部署应用程序时，出现以下错误

10:12:30,321 ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[default-host].[/cq/event-bus]] (ServerService Thread Pool -- 114) JBWEB000289: Servlet com.something.sdlc.rest.application.MyApplication threw load() exception: java.lang.RuntimeException: Unable to instantiate @Form class. No no-arg constructor.
    at org.jboss.resteasy.core.FormInjector.<init>(FormInjector.java:32) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.InjectorFactoryImpl.createParameterExtractor(InjectorFactoryImpl.java:116) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.cdi.CdiInjectorFactory.createParameterExtractor(CdiInjectorFactory.java:51) [resteasy-cdi-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.MethodInjectorImpl.<init>(MethodInjectorImpl.java:42) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.InjectorFactoryImpl.createMethodInjector(InjectorFactoryImpl.java:76) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.cdi.CdiInjectorFactory.createMethodInjector(CdiInjectorFactory.java:57) [resteasy-cdi-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.ResourceMethodInvoker.<init>(ResourceMethodInvoker.java:100) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.ResourceMethodRegistry.processMethod(ResourceMethodRegistry.java:280) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.ResourceMethodRegistry.register(ResourceMethodRegistry.java:251) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.ResourceMethodRegistry.addResourceFactory(ResourceMethodRegistry.java:221) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.ResourceMethodRegistry.addResourceFactory(ResourceMethodRegistry.java:193) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.ResourceMethodRegistry.addResourceFactory(ResourceMethodRegistry.java:179) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.ResourceMethodRegistry.addResourceFactory(ResourceMethodRegistry.java:156) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.core.ResourceMethodRegistry.addPerRequestResource(ResourceMethodRegistry.java:75) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.spi.ResteasyDeployment.registration(ResteasyDeployment.java:430) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.spi.ResteasyDeployment.start(ResteasyDeployment.java:241) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.plugins.server.servlet.ServletContainerDispatcher.init(ServletContainerDispatcher.java:112) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.init(HttpServletDispatcher.java:36) [resteasy-jaxrs-3.0.8.Final.jar:]
    at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1194) [jbossweb-7.4.8.Final-redhat-4.jar:7.4.8.Final-redhat-4]
    at org.apache.catalina.core.StandardWrapper.load(StandardWrapper.java:1100) [jbossweb-7.4.8.Final-redhat-4.jar:7.4.8.Final-redhat-4]
    at org.apache.catalina.core.StandardContext.loadOnStartup(StandardContext.java:3591) [jbossweb-7.4.8.Final-redhat-4.jar:7.4.8.Final-redhat-4]
    at org.apache.catalina.core.StandardContext.start(StandardContext.java:3798) [jbossweb-7.4.8.Final-redhat-4.jar:7.4.8.Final-redhat-4]
    at org.jboss.as.web.deployment.WebDeploymentService.doStart(WebDeploymentService.java:161) [jboss-as-web-7.4.0.Final-redhat-19.jar:7.4.0.Final-redhat-19]
    at org.jboss.as.web.deployment.WebDeploymentService.access$000(WebDeploymentService.java:59) [jboss-as-web-7.4.0.Final-redhat-19.jar:7.4.0.Final-redhat-19]
    at org.jboss.as.web.deployment.WebDeploymentService$1.run(WebDeploymentService.java:94) [jboss-as-web-7.4.0.Final-redhat-19.jar:7.4.0.Final-redhat-19]
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [rt.jar:1.8.0_77]
    at java.util.concurrent.FutureTask.run(FutureTask.java:266) [rt.jar:1.8.0_77]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [rt.jar:1.8.0_77]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [rt.jar:1.8.0_77]
    at java.lang.Thread.run(Thread.java:745) [rt.jar:1.8.0_77]
    at org.jboss.threads.JBossThread.run(JBossThread.java:122)

为了解决这个问题，我必须包含无参数构造函数，如下所示。

public EndpointParams () {
}

一旦我将其包含在我的 POJO 中，部署就会成功。我想知道的是，为什么我需要显式包含无参数构造函数，即使我没有在整个应用程序中使用它？我的应用程序中还有其他几个 POJO，其中我未包含无参数构造函数，并且一切正常。我想知道在什么情况下，我应该显式指定无参数构造函数。

PS:我正在使用 RestEasy、CDI、JBOSS。

Answer 1

这就是 @Form/@BeanParam 的工作原理。 RESTEasy 将尝试使用无参数来构造它，然后填充属性。它不知道如何调用您的自定义构造函数。它不像其他服务(使用 CDI)或什至反序列化(例如 Jackson)那样经历相同的 DI 系统。使用这两者中的任何一个，都允许创建自定义构造函数。但这些是不同的系统。