Question

我想只获得一个POJO的字段名称，我的java POJO看起来像：

public class A1 {

        private String field2;
    private String field1;
    public String getField2() {
        return field2;
    }
    public void setField2(String field2) {
        this.field2 = field2;
    }
    public String getField1() {
        return field1;
    }
    public void setField1(String field1) {
        this.field1 = field1;
    }
    @Override
    public String toString() {
        return "A1 [field2=" + field2 + ", field1=" + field1 + "]";
    }

        public String getFiled1Name() throws NoSuchFieldException, SecurityException{
            return this.getClass().getDeclaredField(field1).getName();
        }

    }

在客户端代码中：

System.out.println(new A1().getFiled1Name());

我正在NullPointerException

我尝试了方法this.getClass().getDeclaredFields()，这可以按预期工作。但问题是：它返回所有字段（java.lang.reflect.Field），我需要迭代，比较然后返回正确的字段名称。

相反，我想只获得一个字段名称，并且方法中没有硬编码字段名称。我怎么能得到这个？

Answer 1

您应该提供字段的名称而不是字段本身，因为field1是字符串，null并且您将null传递给getDeclaredField，

public String getFiled1Name() throws NoSuchFieldException, SecurityException {
    return getClass().getDeclaredField("field1").getName();
}

Class#getDeclareField抛出NullPointerException - 如果name为null

我不想硬编码“field1”

您可以为字段名称创建常量而不是使用反射，但是对于名称中的每个更改，您还需要更新常量。

在其他方面，您可以获取所有声明的字段并从数组中访问字段的名称。但是在使用它时要小心，因为声明中的更改可能会改变方法的行为。

public String getFiled1Name() throws NoSuchFieldException, SecurityException {
    return getClass().getDeclaredFields()[0].getName();
}

Answer 2

您需要将字段名称作为字符串传递。

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
    <link rel="stylesheet"  href="/css/theme.css">
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <meta http-equiv="Content-Style-Type" content="text/css">
  <title></title>
  <meta name="Generator" content="Cocoa HTML Writer">
  <meta name="CocoaVersion" content="1404.46">
  <style type="text/css">
    p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; line-height: 14.0px; font: 12.0px Times; color: #000000; -webkit-text-stroke: #000000}
    p.p3 {margin: 0.0px 0.0px 0.0px 0.0px; line-height: 14.0px; font: 12.0px Times; color: #000000; -webkit-text-stroke: #000000; min-height: 14.0px}
    p.p5 {margin: 0.0px 0.0px 12.0px 0.0px; line-height: 14.0px; font: 12.0px Times; color: #000000; -webkit-text-stroke: #000000}
    p.p7 {margin: 0.0px 0.0px 12.0px 0.0px; line-height: 14.0px; font: 12.0px Times; color: #000000; -webkit-text-stroke: #000000; min-height: 14.0px}
    p.p9 {margin: 0.0px 0.0px 0.0px 0.0px; line-height: 13.0px; font: 11.0px '.AppleSystemUIFont'; color: #000000; -webkit-text-stroke: #000000}
    li.li2 {margin: 0.0px 0.0px 0.0px 0.0px; line-height: 14.0px; font: 12.0px Times; color: #0000ee; -webkit-text-stroke: #0000ee}
    span.s1 {font-kerning: none}
    span.s2 {-webkit-text-stroke: 0px #000000}
    span.s3 {text-decoration: underline ; font-kerning: none}
    span.s4 {font-kerning: none; color: #0000ee; -webkit-text-stroke: 0px #0000ee}
    span.s5 {font-kerning: none; background-color: #c0c0c0}
    span.s6 {font: 12.0px Times; font-kerning: none}
    ul.ul1 {list-style-type: disc}
    
  </style>
</head>
<body>
<p class="p1"><span class="s1"></span></p>
<ul class="ul1">
  <li class="li2"><span class="s2"></span><span class="s3">What we do</span></li>
  <li class="li2"><span class="s2"></span><span class="s3">Our process<span class="Apple-converted-space"> </span></span></li>
  <li class="li2"><span class="s2"></span><span class="s3">Our team<span class="Apple-converted-space"> </span></span></li>
  <li class="li2"><span class="s2"></span><span class="s3">Contact us</span></li>
</ul>
<p class="p3"><span class="s1"></span><br></p>
<h1 style="margin: 0.0px 0.0px 16.1px 0.0px; line-height: 28.0px; font: 24.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Change The Way You Pay<span class="Apple-converted-space"> </span></b></span></h1>
<p class="p5"><span class="s1">With Tip Me, you will be able to make transactions like never before</span></p>
<p class="p1"><span class="s1"></span></p>
<h2 style="margin: 0.0px 0.0px 14.9px 0.0px; line-height: 22.0px; font: 18.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Tip Anyone. Anytime. Anywhere.<span class="Apple-converted-space"> </span></b></span></h2>
<p class="p5"><span class="s1">No need to pull out your wallet anymore. With Tip Me, you will be able to make simple, efficient transactions with the click of a button.<span class="Apple-converted-space"> </span></span></p>
<p class="p1"><span class="s1"></span></p>
<h2 style="margin: 0.0px 0.0px 14.9px 0.0px; line-height: 22.0px; font: 18.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Efficiently Manage Your Money<span class="Apple-converted-space"> </span></b></span></h2>
<p class="p5"><span class="s1">Never lose track of your transactions. You will be able to view who has tipped you, whom you have tipped, and how much was exchanged.</span></p>
<p class="p1"><span class="s1"></span></p>
<h2 style="margin: 0.0px 0.0px 14.9px 0.0px; line-height: 22.0px; font: 18.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Make Secure Transactions<span class="Apple-converted-space"> </span></b></span></h2>
<p class="p5"><span class="s1">Never worry about securing confidential information again. Through Tip Me, all user payments are Level 1 PCI compliance encrypted.</span></p>
<p class="p7"><span class="s1"></span><br></p>
<h1 style="margin: 0.0px 0.0px 16.1px 0.0px; line-height: 28.0px; font: 24.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Our Process<span class="Apple-converted-space"> </span></b></span></h1>
<p class="p5"><span class="s1">Select a friend, enter an amount, and tip. Yes, it's that simple.<span class="Apple-converted-space"> </span></span></p>
<p class="p1"><span class="s1"><span class="Apple-converted-space"> </span></span></p>
<p class="p1"><span class="s1"></span></p>
<p class="p1"><span class="s1"><span class="Apple-converted-space"> </span></span></p>
<p class="p1"><span class="s1"><span class="Apple-converted-space"> </span></span></p>
<p class="p1"><span class="s1"><span class="Apple-converted-space"> </span></span></p>
<h1 style="margin: 0.0px 0.0px 16.1px 0.0px; line-height: 28.0px; font: 24.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Our Team<span class="Apple-converted-space"> </span></b></span></h1>
<p class="p1"><span class="s1"></span></p>
<h2 style="margin: 0.0px 0.0px 14.9px 0.0px; line-height: 22.0px; font: 18.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Nilay Neeranjun<span class="Apple-converted-space"> </span></b></span></h2>
<p class="p5"><span class="s1">Co-Founder, Software Developer</span></p>
<p class="p1"><span class="s4"><a href="file:///Users/nneeranjun/Desktop/Tip%20Me%20Website/www.linkedin.com/in/nilay-neeranjun"></a></span><span class="s1"><span class="Apple-converted-space"> </span></span></p>
<p class="p1"><span class="s1"></span></p>
<h2 style="margin: 0.0px 0.0px 14.9px 0.0px; line-height: 22.0px; font: 18.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Badri Narayan</b></span></h2>
<p class="p5"><span class="s1">Co-Founder, Software Developer</span></p>
<p class="p1"><span class="s4"><a href="https://www.linkedin.com/in/badrinnarayan"></a></span><span class="s1"><span class="Apple-converted-space"> </span></span></p>
<h1 style="margin: 0.0px 0.0px 14.9px 0.0px; line-height: 22.0px; font: 18.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Contact Us</b></span></h1>
<h4 style="margin: 0.0px 0.0px 16.0px 0.0px; line-height: 14.0px; font: 12.0px Times; color: #000000; -webkit-text-stroke: #000000"><span class="s1"><b>Send us a message and we will get back to you as soon as possible</b></span></h4>
<p class="p7"><span class="s1"></span><br></p>
<p class="p7"><span class="s1"></span><br></p>
<p class="p7"><span class="s1"></span><br></p>
<p class="p7"><span class="s1"></span><br></p>
<p class="p3"><span class="s1"></span><br></p>
<p class="p3"><span class="s1"></span><br></p>
<p class="p9"><span class="s5">Submit</span><span class="s6"><span class="Apple-converted-space"> </span></span></p>
<p class="p1"><span class="s1">Copyright © TipMe 2015<span class="Apple-converted-space"> </span></span></p>
</body>
</html>

Answer 3

您的字段 field1 为空。您尚未初始化String。这会导致NullPointerException。您目前只是将 null 传递给 getDeclaredField（）方法。

Answer 4

我想到了这一点，不确定Thread.currentThread().getStackTrace()是否可以接受，但是你想要一些完全动态的东西，没有硬编码......所以这是我的拍摄：

public class Reflections {

    private String field2;
    private String field1;

    public static void main(String[] args) {
        Reflections r = new Reflections();
        System.out.println(r.getField1Name());
    }

    private Field fields[] = null;

    public Reflections() {
        fields = getClass().getDeclaredFields();
    }

    public String getField1Name() {

        if(fields == null || fields.length==0){
            return null;
        }

        final StackTraceElement[] stes = Thread.currentThread().getStackTrace();
        if(stes == null || stes.length<2){
            return null;
        }

        final String thisMethodName = stes[1].getMethodName(); // [1] this is supposed to have the name of current method

        for (Field f : fields) {
            if (thisMethodName.toLowerCase().contains(f.getName().toLowerCase())) {
                //System.out.println(f.getName());
                return f.getName();
            }// match
        }// for fields
        return null;
    }// getFiled1Name   
}

简而言之，getField1Name()试图找到它自己的名字，然后遍历当前类的字段列表，并尝试查找其名称是方法名称的一部分的字段。

这有时可能会返回无效结果，因为某些字段名称可能是较长名称的子部分，以及其他一些结果不准确的情况

但是，在这里它可能对你或其他任何人都有用。

你可以重构代码，创建新方法getFieldNameUsingMethodName() ??也许？！？！这将做循环和匹配的事情，如果你有10个或更多getFieldXname()所以你可以这样做，如果你认为这个溶胶值得。

使用this.getClass（）时的NullPointerException .getDeclaredField方法

4 个答案: