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本篇文章介绍了 Java 中的 java.net.UnknownHostException 错误。

Java中java.net.UnknownHostException的原因

java.net.UnknownHostException 表示找不到主机名的 IP 地址。 通常，当主机名输入错误时会发生这种情况。

此异常的另一个原因是 DNS 配置错误或 DNS 传播。 新的 DNS 可能需要 48 小时才能在整个互联网上传播。

下面是 Java 中 UnknownHostException 的示例：

package jiyik;

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

public class Example {
    public static void main(String[] args) {
        String Host_Name = "https://delftstack.co";
        URL Demo_Url = null;

        try {
            Demo_Url = new URL(Host_Name);
            HttpURLConnection Demo_Connection = (HttpURLConnection) Demo_Url.openConnection();
            System.out.println(Demo_Connection.getResponseCode());
        } catch (MalformedURLException me) {
            me.printStackTrace();
        } catch (IOException ie) {
            ie.printStackTrace();
        }
    }
}

上面尝试连接到主机，但主机名有输入错误。 它将抛出 UnknownHostException。

查看输出：

java.net.UnknownHostException: delftstack.co
    at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:567)
    at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:327)
    at java.base/java.net.Socket.connect(Socket.java:633)
    at java.base/sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:299)
    at java.base/sun.security.ssl.BaseSSLSocketImpl.connect(BaseSSLSocketImpl.java:174)
    at java.base/sun.net.NetworkClient.doConnect(NetworkClient.java:183)
    at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:498)
    at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:603)
    at java.base/sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:266)
    at java.base/sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:380)
    at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:189)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1242)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1128)
    at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:175)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1665)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1589)
    at java.base/java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:529)
    at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.getResponseCode(HttpsURLConnectionImpl.java:308)
    at jiyik.Example.main(Example.java:16)

java.net.UnknownHostException 是一个已检查异常，可以使用 try-catch 块进行处理。 请参阅下面如何处理 Java 中的 UnknownHostException：

package jiyik;

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.UnknownHostException;

public class Example {
    public static void main(String[] args) {
        String Host_Name = "https://delftstack.co";
        URL Demo_Url = null;

        try {
            Demo_Url = new URL(Host_Name);
            HttpURLConnection Demo_Connection = (HttpURLConnection) Demo_Url.openConnection();
            System.out.println(Demo_Connection.getResponseCode());
        } catch (UnknownHostException ue) {
            ue.printStackTrace();
        } catch (MalformedURLException me) {
            me.printStackTrace();
        } catch (IOException ie) {
            ie.printStackTrace();
        }
    }
}

防止Java中的java.net.UnknownHostException

为了防止Java中出现 java.net.UnknownHostException，我们应该考虑以下两点：

1. 首先，确保主机名正确并且没有任何输入错误 - 仔细检查是否有任何输入错误或空格。
2. 检查 DNS 设置以确保服务器可访问。 新主机名需要一段时间才能跟上 DNS 服务器的速度。

让我们使用正确的主机名尝试相同的示例。 参见示例：

package jiyik;

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.UnknownHostException;

public class Example {
    public static void main(String[] args) {
        String Host_Name = "http://localhost/";
        URL Demo_Url = null;

        try {
            Demo_Url = new URL(Host_Name);
            HttpURLConnection Demo_Connection = (HttpURLConnection) Demo_Url.openConnection();
            System.out.println(Demo_Connection.getResponseCode());
        } catch (UnknownHostException uhe) {
            uhe.printStackTrace();
        } catch (MalformedURLException me) {
            me.printStackTrace();
        } catch (IOException ie) {
            ie.printStackTrace();
        }
    }
}

上面的代码将正常工作并打印响应代码。 查看输出：

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