表单与 JSON：同一组字段为什么发送结果不同

import java.io.ByteArrayOutputStream;
import java.nio.ByteBuffer;
import java.nio.charset.*;
import java.util.*;
public class Main {
  static String component(String raw) {
    StringBuilder out=new StringBuilder();
    for(int i=0;i<raw.length();) {
      char ch=raw.charAt(i);
      if(ch>127) throw new IllegalArgumentException("非 ASCII 必须编码");
      if(ch=='+') { out.append(' '); i++; continue; }
      if(ch!='%') { out.append(ch); i++; continue; }
      ByteArrayOutputStream bytes=new ByteArrayOutputStream();
      while(i<raw.length() && raw.charAt(i)=='%') {
        if(i+2>=raw.length()) throw new IllegalArgumentException("百分号不完整");
        int hi=Character.digit(raw.charAt(i+1),16),lo=Character.digit(raw.charAt(i+2),16);
        if(hi<0 || lo<0 || raw.charAt(i+1)>127 || raw.charAt(i+2)>127)
          throw new IllegalArgumentException("非法十六进制");
        bytes.write(hi*16+lo); i+=3; // 收集完整连续字节再解码
      }
      try {
        CharsetDecoder decoder=StandardCharsets.UTF_8.newDecoder()
          .onMalformedInput(CodingErrorAction.REPORT).onUnmappableCharacter(CodingErrorAction.REPORT);
        out.append(decoder.decode(ByteBuffer.wrap(bytes.toByteArray())));
      } catch(CharacterCodingException e) { throw new IllegalArgumentException("非法 UTF-8",e); }
    }
    return out.toString();
  }
  static Map<String,List<String>> parse(String body) {
    if(body==null || body.length()>4096) throw new IllegalArgumentException();
    Map<String,List<String>> result=new LinkedHashMap<>();
    for(String pair:body.split("&",-1)) {
      if(pair.isEmpty()) continue;
      int split=pair.indexOf('='); // 先结构切分，后组件解码
      String key=component(split<0?pair:pair.substring(0,split));
      String value=component(split<0?"":pair.substring(split+1));
      result.computeIfAbsent(key,k->new ArrayList<>()).add(value);
    }
    return result;
  }
  static void eq(Object wanted,Object actual) { if(!Objects.equals(wanted,actual)) throw new AssertionError(actual); }
  static void reject(String body) {
    try { parse(body); throw new AssertionError("应拒绝"); }
    catch(IllegalArgumentException expected) { /* 期望错误 */ }
  }
  public static void main(String[] args) {
    // 固定期望覆盖正常、空值和错误边界，失败立即中断验证
    eq(List.of("java","web"),parse("tag=java&tag=web").get("tag"));
    eq(List.of("qing huan"),parse("name=qing+huan").get("name"));
    eq(List.of("中文"),parse("x=%E4%B8%AD%E6%96%87").get("x"));
    eq(Map.of(),parse(""));
    eq(List.of("x"),parse("=x").get(""));
    eq(List.of("&a=b"),parse("x=%26a%3Db").get("x"));
    eq(List.of("%26"),parse("x=%2526").get("x"));
    reject("x=%GG"); reject("x=%C3");
    System.out.println("PASS 9 form tests");
  }
}