java.lang.NullPointerException 在 java.net.URI 错误

Question

我正在尝试通过 Java 访问一些 URL。

在我的代码中，我提供了用于服务器身份验证的凭据。我还提取了服务器上可用 URL 的列表（以 xml 形式），并将它们放入一个数组中，我可以在其中自动调用列表中的每个 URL。

这是我的代码：

import java.io.File;
import org.apache.http.auth.AuthScope;
import org.apache.http.auth.UsernamePasswordCredentials;
import org.apache.http.client.CredentialsProvider;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.BasicCredentialsProvider;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;
import java.io.FileOutputStream;
import java.lang.reflect.Array;
import org.apache.http.HttpEntity;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.NodeList;
import org.w3c.dom.Node;

public class ClientAuthentication {

public static void main(String[] args) throws Exception {
    /*CREATING FILE*/
    File myFile = new File("C:/input.xml");

    /*DEFINING CREDENTIALS*/
    CredentialsProvider credsProvider = new BasicCredentialsProvider();
    credsProvider.setCredentials(new AuthScope("localhost", 80),new UsernamePasswordCredentials(" ", " "));

    try (CloseableHttpClient httpclient = HttpClients.custom().setDefaultCredentialsProvider(credsProvider).build()){
        HttpGet httpget = new HttpGet("http://localhost/app/rest/buildTypes");
        /*WRITE TO FILE*/
        try (CloseableHttpResponse response = httpclient.execute(new HttpGet("http://localhost/app/rest/buildTypes"))){
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                try (FileOutputStream outstream = new FileOutputStream(myFile)) {
                entity.writeTo(outstream);
                }
            }
        }

        /*SHOW EXECUTION*/
        System.out.println("Executing request " + httpget.getRequestLine());
        try (CloseableHttpResponse response = httpclient.execute(httpget)) {
            System.out.println("----------------------------------------");
            System.out.println(response.getStatusLine());
            System.out.println(EntityUtils.toString(response.getEntity()));
        }
    }

    /*Extract all Build ID from XML file*/
    DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = factory.newDocumentBuilder();
    Document doc = builder.parse("C:/input.xml");
    XPathFactory xPathfactory = XPathFactory.newInstance();
    XPath xpath = xPathfactory.newXPath();
    XPathExpression expr = xpath.compile("//buildTypes/buildType[@id]");
    NodeList nl = (NodeList) expr.evaluate(doc, XPathConstants.NODESET);

    /*Creating url and saving into array*/
    String array[] = new String[100];
    for (int i = 0; i < nl.getLength(); i++){
        Node currentItem = nl.item(i);
        String key = currentItem.getAttributes().getNamedItem("id").getNodeValue();
        String pathing = "http://localhost/app/rest/" + (key);
        System.out.println(pathing);
        array[i] = pathing;
    }

    /*Getting the url*/
    int size = Array.getLength(array);
    for (int i = 0; i < size; i++){
        try (CloseableHttpClient areq = HttpClients.custom().setDefaultCredentialsProvider(credsProvider).build()){
                HttpGet httpget = new HttpGet(array[i]);

                /*SHOW EXECUTION*/
                System.out.println("Executing request " + httpget.getRequestLine());
                try (CloseableHttpResponse response = areq.execute(httpget)) {
                    System.out.println("----------------------------------------");
                    System.out.println(response.getStatusLine());
                    System.out.println(EntityUtils.toString(response.getEntity()));
                }
            }
    }
}  
}

输出错误为：

Exception in thread "main" java.lang.NullPointerException
    at java.net.URI$Parser.parse(URI.java:3042)
    at java.net.URI.<init>(URI.java:588)
    at java.net.URI.create(URI.java:850)
    at org.apache.http.client.methods.HttpGet.<init>(HttpGet.java:66)
    at ClientAuthentication.main(ClientAuthentication.java:85)
Java returned: 1 BUILD FAILED (total time: 0 seconds)

我认为问题出在代码的最后部分（获取 URL）。我不知道问题是什么。有人可以帮忙吗？

Answer 1

String array[] = new String[100];
for (int i = 0; i < nl.getLength(); i++){
    ...

您为什么使用 100 而不是 nl.getLength？

由于您稍后迭代此数组以实例化 HttpGet，每个未实例化的最后一个单元格仍然是 null，导致 new HttpGet(array[i]) 抛出异常，因为参数不能null.

int size = Array.getLength(array);
for (int i = 0; i < size; i++){
    try (CloseableHttpClient areq = HttpClients.custom().setDefaultCredentialsProvider(credsProvider).build()){
            HttpGet httpget = new HttpGet(array[i]);

这将一直有效，直到您达到阵列中剩余的 null。

使用 new String[nl.getLength()] 实例化您的数组。既然你现在不需要更多。（或更少）。

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