在JDK中，當使用DataOutputStream.writeBytes方法傳遞中文時，會出現亂碼的問題。

java 项目中有段封装的发送 post 请求方法

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Map;
import java.util.Map.Entry;

public class FormDataPostRequest {

    public static String sendPostFormData(String url, Map<String, String> headerParams, String text) {
        String result = ""; // 返回的结果
        BufferedReader in = null; // 读取响应输入流

        try {
            //创建连接
            URL apiUrl = new URL(url);
            HttpURLConnection connection = (HttpURLConnection) apiUrl.openConnection();
            connection.setDoOutput(true);
            connection.setDoInput(true);
            connection.setRequestMethod("POST");
            connection.setUseCaches(false);
            connection.setInstanceFollowRedirects(true);
            connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + "*****"); // 设置boundary
            if (headerParams != null) {
                for (Entry<String, String> entry : headerParams.entrySet()) {
                    connection.setRequestProperty(entry.getKey(), entry.getValue());
                }
            }
            connection.connect();

            // 发送POST请求，包含字符串参数
            DataOutputStream out = new DataOutputStream(connection.getOutputStream());
            // 添加字符串参数
            out.writeBytes("--*****\r\n");
            out.writeBytes("Content-Disposition: form-data; name=\"text\"\r\n\r\n");
            out.write(text.getBytes("UTF-8"));
            out.writeBytes("\r\n");
            out.writeBytes("--*****--\r\n");
            out.flush();
            out.close();

            // 读取响应
            // 定义BufferedReader输入流来读取URL的响应,并设置编码方式
            in = new BufferedReader(new InputStreamReader(connection.getInputStream(), "UTF-8"));
            String line;
            // 读取返回的内容
            while ((line = in.readLine()) != null) {
                result += line;
            }
        } catch (Exception e) {
            e.printStackTrace();
            System.out.println("Http请求方法内部问题");
        } finally {
            try {
                if (in != null) {
                    in.close();
                }
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
        return result;
    }
}

原本使用没有什麼問題，後續在使用過程中，發現當參數的值為中文時，接收方會收到亂碼，查看程式碼發現問題出現這一行。

out.writeBytes(text + "\r\n");

查看writeBytes原始碼

/**
 * Writes out the string to the underlying output stream as a
 * sequence of bytes. Each character in the string is written out, in
 * sequence, by discarding its high eight bits. If no exception is
 * thrown, the counter {@code written} is incremented by the
 * length of {@code s}.
 *
 * @param      s   a string of bytes to be written.
 * @throws     IOException  if an I/O error occurs.
 * @see        java.io.FilterOutputStream#out
 */
public final void writeBytes(String s) throws IOException {
    int len = s.length();
    for (int i = 0 ; i < len ; i++) {
        out.write((byte)s.charAt(i));
    }
    incCount(len);
}

發現字串 s 的每個字元都會被強制轉換成 byte 型別，byte 型別是 8 位元的，而中文的 char 是 16 位元，所以當字元為中文時，會被截斷，導致亂碼，可以加一些日誌，看一下轉換後的 byte 陣列。

public final void writeBytes(String s) throws IOException {
    int len = s.length();
    for (int i = 0 ; i < len ; i++) {
        char c = s.charAt(i);
        byte b = (byte) c;
        System.out.println("字元：" + c + " 轉換後的byte陣列：" + b);
        out.write(b);
    }
    incCount(len);
}

最終能看到中文字元被強轉後，會丟失 8 位元，導致亂碼。因此解決方案將字串轉換成 byte 陣列，然後寫入輸出流。

// 發送POST請求，包含字串參數
DataOutputStream out = new DataOutputStream(connection.getOutputStream());
// 添加字串參數
out.writeBytes("--*****\r\n");
out.writeBytes("Content-Disposition: form-data; name=\"text\"\r\n\r\n");
out.write(text.getBytes("UTF-8"));
out.writeBytes("\r\n");
out.writeBytes("--*****--\r\n");
out.flush();
out.close();