- 客户端没有访问该资源的权限
- 客户端要访问的项目不存在
- 等等

遇到这些情况时，通常要返回 **4XX**（400 至 499）**HTTP 状态码**。

这与表示请求成功的 **2XX**（200 至 299）HTTP 状态码类似。那些“200”状态码表示某种程度上的“成功”。

而 **4XX** 状态码表示客户端发生了错误。

大家都知道**「404 Not Found」**错误，还有调侃这个错误的笑话吧？

## 使用 `HTTPException` { #use-httpexception }

向客户端返回 HTTP 错误响应，可以使用 `HTTPException`。

### 导入 `HTTPException` { #import-httpexception }

// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package tar

import (
	"bytes"
	"fmt"
	"strconv"
	"strings"
	"time"
)

// hasNUL reports whether the NUL character exists within s.
func hasNUL(s string) bool {
	return strings.Contains(s, "\x00")
}

// isASCII reports whether the input is an ASCII C-style string.

        return "form";
    }

    /**
     * Gets the parameter value for the specified name.
     * @param name The parameter name.
     * @return The parameter value, or null if not found.
     */
    public String getParameter(final String name) {
        return parameterMap.get(name);
    }

    @Override
    public String getRealm() {
        return null;
    }

    /*
     * uncheckedCastNullableTToT is safe as long as the callers are careful to pass a "real" T
     * whenever they pass `true` for the matching `has*Bound` parameter.
     */
    if (hasLowerBound) {
      int unused =
          comparator.compare(
              uncheckedCastNullableTToT(lowerEndpoint), uncheckedCastNullableTToT(lowerEndpoint));
    }

   * Range#contains contained} by the range.
   *
   * @throws IllegalArgumentException if neither range nor the domain has a lower bound, or if
   *     neither has an upper bound
   * @since 13.0
   */
  public static <C extends Comparable> ContiguousSet<C> create(
      Range<C> range, DiscreteDomain<C> domain) {
    checkNotNull(range);
    checkNotNull(domain);

        PreauthIntegrityContext context = sessionContexts.get(sessionId);
        if (context == null) {
            if (enforceIntegrity) {
                throw new CIFSException("No preauth integrity context found for session: " + sessionId);
            }
            log.warn("No preauth integrity context for session {}, skipping update", sessionId);
            return;
        }

        if (!context.isValid()) {

            assertNotNull(ticket);
        }
    }

    @Test
    void testConstructorWithKeyNotFound() throws IOException {
        // Test when no suitable Kerberos key is found
        byte[] token = createTestTicketBytes(new BigInteger(KerberosConstants.KERBEROS_VERSION), SERVER_REALM, SERVER_PRINCIPAL_NAME,
                ENCRYPTION_TYPE, ENCRYPTED_DATA, null);

				}
			}
		default:
			bts, err = msgp.Skip(bts)
			if err != nil {
				err = msgp.WrapError(err)
				return
			}
		}
	}
	o = bts
	return
}

// Msgsize returns an upper bound estimate of the number of bytes occupied by the serialized message
func (z *nsScannerOptions) Msgsize() (s int) {
	s = 1 + 3 + msgp.StringPrefixSize + len(z.DiskID) + 2 + msgp.IntSize + 2
	if z.Cache == nil {

                        saveBuffer();
                        const promptStart = line.indexOf(promptLiteralStart);
                        if (promptStart === -1) {
                            console.error("Custom prompt found but no end delimiter", line)
                        }
                        const prompt = line.slice(0, promptStart).replace(customPromptLiteralStart, "")

		zb0001, bts, err = msgp.ReadUintBytes(bts)
		if err != nil {
			err = msgp.WrapError(err)
			return
		}
		(*z) = BitrotAlgorithm(zb0001)
	}
	o = bts
	return
}

// Msgsize returns an upper bound estimate of the number of bytes occupied by the serialized message
func (z BitrotAlgorithm) Msgsize() (s int) {
	s = msgp.UintSize
	return
}

// DecodeMsg implements msgp.Decodable