在 Struts2 中登录期间验证获取 NullPointerException 的方法

Question

I am doing programmatic validation, when I click the submit button, the validate method has been called, but it shows NullPointerException . It won't validate, but I am unable to find my mistake.

---

index.jsp:

<%@ taglib uri="/struts-tags" prefix="s"%>

<s:actionerror />
<s:form action="insert">
    <s:textfield name="" key="user.name" />
    <s:textfield name="" key="user.pass" />
    <s:submit value="login" />
</s:form>

---

UserLogin.java:

package jack;

import com.opensymphony.xwork2.ActionSupport;

public class UserLogin extends ActionSupport {

private String username;
private String password;

//getter and setter method

@Override
public void validate() {
    System.out.println("validate");
    if (this.username.equals("") || username.length() == 0) {
        this.addFieldError(username, getText("user.wrong"));
    }
    if (this.password.equals("") || password.length() == 0) {
        this.addFieldError(password, getText("pass.wrong"));
    }
}

@Override
public String execute() throws Exception {
    System.out.println("execute");
    return SUCCESS;
}
}

---

struts.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE struts PUBLIC
"-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
"http://struts.apache.org/dtds/struts-2.0.dtd">

<struts>
    <package name="a" extends="struts-default">
        <action name="insert" class="jack.UserLogin">
            <result name="success">/success.jsp</result>
            <result name="error">/error.jsp</result>
            <result name="input">/index.jsp</result>
        </action>
    </package>
</struts>

---

struts.properties:

 struts.custom.i18n.resources=Mybundle

---

Mybundle.properties:

user.name=username
user.pass=password
user.wrong=enter username
pass.wrong=enter password

Answer 1

Frame work is unable set values for your variables, your jsp should be as below

<s:textfield name="username" key="user.name" />
<s:textfield name="password" key="user.pass" />

Hope this solves your issue.

Answer 2

You have a null pointer exception on line 29 of UserLogin.java. Assuming that this is the line:

if (this.username.equals("") || username.length() == 0) {

It means that username is null. You can't call a method (in this case equals ) of a variable (in this case username ) that is null. You will get exactly the error as described. What's happening is that struts is not properly assigning values to these variables. You need to fix that first.

Answer 3

Before validation check for null in validate method. See below.

if(this.username!=null && this.password != null)