在eclipse上将JSF与Tomcat一起使用时，总是返回一个空指针。这个项目是关于IP跟踪的，需要将另一个bean注入到应用程序bean中。
要注入的Bean：track使用注入的bean：increaseCount
轨道类
import java.util.HashMap;
import java.util.Map;
import javax.enterprise.context.ApplicationScoped;
import javax.inject.Named;
@Named(value = "track")
@ApplicationScoped
public class Track {
private Map<String, Integer> map = new HashMap<>();
public void add(String ipAddress) {
map.put(ipAddress, map.containsKey(ipAddress) ? map.get(ipAddress) + 1 : 1);
}
public int getCount(String ipAddress) {
return map.containsKey(ipAddress) ? map.get(ipAddress) : 0;
}
public String getAllCount() {
return "Count summary is " + map;
}
}
增量计数类
我尝试遵循此处所示的解决方案，但遇到了相同的问题。
httpss://stackoverflow.com/questions/16399974/nullpointerexception-while-trying-to-access-inject-bean-in-constructor
import javax.annotation.PostConstruct;
import javax.enterprise.context.SessionScoped;
import javax.faces.context.FacesContext;
import javax.inject.Inject;
import javax.inject.Named;
import javax.servlet.http.HttpServletRequest;
@Named(value = "increaseCount")
@SessionScoped
public class IncreaseCount implements java.io.Serializable {
@Inject
private Track track;
private String ipAddress;
@PostConstruct
public void init() {
HttpServletRequest request = (HttpServletRequest) FacesContext.getCurrentInstance().getExternalContext()
.getRequest();
this.ipAddress = request.getRemoteAddr();
}
/*
* public IncreaseCount() { HttpServletRequest request = (HttpServletRequest)
* FacesContext.getCurrentInstance().getExternalContext() .getRequest();
* this.ipAddress = request.getRemoteAddr(); }
*/
public void click() {
track.add(ipAddress);
}
public String getIpAddress() {
return ipAddress;
}
public int getCount() {
return track.getCount(ipAddress);
}
}
XHTML页面
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:h="http://xmlns.jcp.org/jsf/html">
<h:head>
<title>IncreaseCount</title>
</h:head>
<h:body>
<h:form>
<h:commandButton action="#{increaseCount.click()}" value="Click Me" />
<br>The current count is #{increaseCount.getCount()} and your IP address is #{increaseCount.getIpAddress()}.</br>
</h:form>
</h:body>
</html>
异常
正如您所看到的，在Track类中调用getCount()方法会产生一个Exception。
Root Cause
java.lang.NullPointerException
chapter39.IncreaseCount.getCount(IncreaseCount.java:39)
java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.base/java.lang.reflect.Method.invoke(Method.java:567)
javax.el.BeanELResolver.invoke(BeanELResolver.java:158)
javax.el.CompositeELResolver.invoke(CompositeELResolver.java:79)
org.apache.el.parser.AstValue.getValue(AstValue.java:159)
org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:190)
com.sun.faces.facelets.el.ELText$ELTextVariable.writeText(Unknown Source)
com.sun.faces.facelets.el.ELText$ELTextComposite.writeText(Unknown Source)
com.sun.faces.facelets.compiler.TextInstruction.write(Unknown Source)
com.sun.faces.facelets.compiler.UIInstructions.encodeBegin(Unknown Source)
com.sun.faces.facelets.compiler.UILeaf.encodeAll(Unknown Source)
javax.faces.render.Renderer.encodeChildren(Unknown Source)
javax.faces.component.UIComponentBase.encodeChildren(Unknown Source)
javax.faces.component.UIComponent.encodeAll(Unknown Source)
javax.faces.component.UIComponent.encodeAll(Unknown Source)
javax.faces.component.UIComponent.encodeAll(Unknown Source)
com.sun.faces.application.view.FaceletViewHandlingStrategy.renderView(Unknown Source)
com.sun.faces.application.view.MultiViewHandler.renderView(Unknown Source)
com.sun.faces.lifecycle.RenderResponsePhase.execute(Unknown Source)
com.sun.faces.lifecycle.Phase.doPhase(Unknown Source)
com.sun.faces.lifecycle.LifecycleImpl.render(Unknown Source)
javax.faces.webapp.FacesServlet.service(Unknown Source)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
For JSF-based bean definitions:-
javax.faces.bean.SessionScoped //for bean scoping
javax.faces.bean.ManagedBean //for bean declaration
javax.faces.bean.ManagedProperty //for bean injection
@ManagedProperty(value="#{yourBeanName}")
private Track track;
为了更好地理解，请浏览此链接