🍃前言

今天我们将对图书管理系统进行收尾工作，今天的开发任务有两个

1. 实现统一数据返回格式
2. 实现统一异常的处理

🎍统一数据返回格式

在【JavaEE进阶】图书管理系统开发日记——柒实现拦截器时，博主对数据的返回格式其实已经进行了封装


但是如果每一个接口都这样写，岂不太麻烦了一点儿。

其实spring boot为我们提供了统一数据格式返回的功能

🚩快速入门

统⼀的数据返回格式使⽤ @ControllerAdvice 和ResponseBodyAdvice 的⽅式实现@ControllerAdvice 表⽰控制器通知类

添加类 ResponseAdvice ,实现 ResponseBodyAdvice 接⼝,并在类上添加@ControllerAdvice 注解

@ControllerAdvice
public class ResponseAdvice implements ResponseBodyAdvice {@Overridepublic boolean supports(MethodParameter returnType, Class converterType) {return true;}@Overridepublic Object beforeBodyWrite(Object body, MethodParameter returnType,MediaType selectedContentType, Class selectedConverterType, ServerHttpRequestrequest, ServerHttpResponse response) {return Result.success(body);}
}

• supports方法:判断是否要执行beforeBodyWrite方法.true为执行,false不执行.通过该⽅法可以选择哪些类或哪些方法的response要进行处理,其他的不进行处理

从returnType获取类名和⽅法名
//获取执⾏的类
Class<?> declaringClass = returnType.getMethod().getDeclaringClass();
//获取执⾏的⽅法
Method method = returnType.getMethod();

• beforeBodyWrite方法:对response方法进行具体操作处理

接下来我们进行测试一下，我们先来看一下没有添加统一功能的时候的登录返回

添加统一功能返回后

但是此时如果运用到我们的项目中，是会出现问题的

🚩存在问题

这里我就不演示错误的情况了

这里直接给出结论，返回结果为String或i为Result类型时会出现错误

🎈问题原因

那么是什么原因造成的呢？

SpringMVC默认会注册⼀些⾃带的 HttpMessageConverter (从先后顺序排列分别为ByteArrayHttpMessageConverter ,
StringHttpMessageConverter , SourceHttpMessageConverter ,SourceHttpMessageConverter,AllEncompassingFormHttpMessageConverter )

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapterimplements BeanFactoryAware, InitializingBean {//...public RequestMappingHandlerAdapter() {this.messageConverters = new ArrayList<>(4);this.messageConverters.add(new ByteArrayHttpMessageConverter());this.messageConverters.add(new StringHttpMessageConverter());if (!shouldIgnoreXml) {try {this.messageConverters.add(new SourceHttpMessageConverter<>());}catch (Error err) {
// Ignore when no TransformerFactory implementation is available}}this.messageConverters.add(newAllEncompassingFormHttpMessageConverter());}
//...
}

其中AllEncompassingFormHttpMessageConverter会根据项⽬依赖情况添加对应的HttpMessageConverter

public AllEncompassingFormHttpMessageConverter() {if (!shouldIgnoreXml) {try {addPartConverter(new SourceHttpMessageConverter<>());}catch (Error err) {
// Ignore when no TransformerFactory implementation is available}if (jaxb2Present && !jackson2XmlPresent) {addPartConverter(new Jaxb2RootElementHttpMessageConverter());}}if (kotlinSerializationJsonPresent) {addPartConverter(new KotlinSerializationJsonHttpMessageConverter());}if (jackson2Present) {addPartConverter(new MappingJackson2HttpMessageConverter());}else if (gsonPresent) {addPartConverter(new GsonHttpMessageConverter());}else if (jsonbPresent) {addPartConverter(new JsonbHttpMessageConverter());}if (jackson2XmlPresent && !shouldIgnoreXml) {addPartConverter(new MappingJackson2XmlHttpMessageConverter());}if (jackson2SmilePresent) {addPartConverter(new MappingJackson2SmileHttpMessageConverter());}
}

在依赖中引⼊jackson包后，容器会把MappingJackson2HttpMessageConverter 自动注册到
messageConverters 链的末尾.

Spring会根据返回的数据类型,从 messageConverters 链选择合适的HttpMessageConverter .

当返回的数据是非字符串时,使用的MappingJackson2HttpMessageConverter 写⼊返回对象.

当返回的数据是字符串时，StringHttpMessageConverter 会先被遍历到，这时会认为StringHttpMessageConverter 可以使用

public abstract class AbstractMessageConverterMethodProcessor extendsAbstractMessageConverterMethodArgumentResolverimplements HandlerMethodReturnValueHandler {//...代码省略protected <T> void writeWithMessageConverters(@Nullable T value,MethodParameter returnType,ServletServerHttpRequest inputMessage, ServletServerHttpResponseoutputMessage)throws IOException, HttpMediaTypeNotAcceptableException,HttpMessageNotWritableException {
//...代码省略if (selectedMediaType != null) {selectedMediaType = selectedMediaType.removeQualityValue();for (HttpMessageConverter<?> converter : this.messageConverters) {GenericHttpMessageConverter genericConverter = (converterinstanceof GenericHttpMessageConverter ?(GenericHttpMessageConverter<?>) converter : null);if (genericConverter != null ?((GenericHttpMessageConverter)converter).canWrite(targetType, valueType, selectedMediaType) :converter.canWrite(valueType, selectedMediaType)) {
//getAdvice().beforeBodyWrite 执⾏之后, body转换成了Result类型的结果body = getAdvice().beforeBodyWrite(body, returnType,selectedMediaType,(Class<? extends HttpMessageConverter<?>>)converter.getClass(),inputMessage, outputMessage);if (body != null) {Object theBody = body;LogFormatUtils.traceDebug(logger, traceOn ->"Writing [" + LogFormatUtils.formatValue(theBody,!traceOn) + "]");addContentDispositionHeader(inputMessage, outputMessage);if (genericConverter != null) {genericConverter.write(body, targetType,selectedMediaType, outputMessage);}else {
//此时cover为StringHttpMessageConverter((HttpMessageConverter) converter).write(body,selectedMediaType, outputMessage);}}else {if (logger.isDebugEnabled()) {logger.debug("Nothing to write: null body");}}return;}}}
//...代码省略}
//...代码省略
}

在 ((HttpMessageConverter) converter).write(body, selectedMediaType,outputMessage) 的处理中,调用父类的write方法

由于 StringHttpMessageConverter 重写了addDefaultHeaders方法,所以会执行⼦类的⽅法

然⽽⼦类 StringHttpMessageConverter 的addDefaultHeaders⽅法定义接收参数为String,此时t为Result类型,所以出现类型不匹配"Result cannot be cast to java.lang.String"的异常

🎈代码修改

如果⼀些⽅法返回的结果已经是Result类型了,那就直接返回Result类型的结果即可

如果是String类型，进行处理一下即可

@ControllerAdvice
public class ResponseAdvice implements ResponseBodyAdvice {@Autowiredprivate ObjectMapper objectMapper;@Overridepublic boolean supports(MethodParameter returnType, Class converterType) {return true;}@SneakyThrows@Overridepublic Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {//在返回之前, 需要做的事情//body 是返回的结果if (body instanceof Result){return body;}if (body instanceof String){return objectMapper.writeValueAsString(Result.success(body));}return Result.success(body);}
}

🚩统一格式返回的优点

1. ⽅便前端程序员更好的接收和解析后端数据接⼝返回的数据
2. 降低前端程序员和后端程序员的沟通成本,按照某个格式实现就可以了,因为所有接⼝都是这样返回的.
3. 有利于项⽬统⼀数据的维护和修改.
4. 有利于后端技术部⻔的统⼀规范的标准制定,不会出现稀奇古怪的返回内容

🍀统一异常处理

统⼀异常处理使⽤的是 @ControllerAdvice+@ExceptionHandler 来实现的,

• @ControllerAdvice 表⽰控制器通知类
• @ExceptionHandler 是异常处理器，

两个结合表示当出现异常的时候执行某个通知，也就是执行某个方法事件

简单使用如下：

@ControllerAdvice
@ResponseBody
public class ErrorAdvice {@ExceptionHandlerpublic Object handler(Exception e) {return Result.fail(e.getMessage());}
}

以上代码表⽰，如果代码出现Exception异常(包括Exception的⼦类),就返回⼀个Result的对象,

Result对象的设置参考博主对返回消息的包装Result.fail(e.getMessage())

public static Result fail(String msg) {Result result = new Result();result.setStatus(ResultStatus.FAIL);result.setErrorMessage(msg);result.setData("");return result;
}

现在我们将它应用于项目中，我们需要针对不同的异常进行返回不同的结果

@ResponseBody
@ControllerAdvice
public class ErrorAdvice {@ExceptionHandlerpublic Object handler(Exception e) {return Result.fail(e.getMessage());}@ExceptionHandlerpublic Object handler(NullPointerException e) {return Result.fail("发⽣NullPointerException:"+e.getMessage());}@ExceptionHandlerpublic Object handler(ArithmeticException e) {return Result.fail("发⽣ArithmeticException:"+e.getMessage());}
}

至此我们的图书管理系统后端开发已经完毕了，接下来由于我们统一了数据返回格式，所以我们需要对前端代码进行修改

🌲前端代码的修改

🚩登录页面

登录界⾯没有拦截,只是返回结果发⽣了变化,所以只需要根据返回结果修改对应代码即可

登录结果代码修改

function login() {$.ajax({type: "post",url: "/user/login",data: {name: $("#userName").val(),password: $("#password").val()},success: function (result) {if (result.status=="SUCCESS" && result.data==true) {location.href = "book_list.html";} else {alert("账号或密码不正确!");}}});
}

🚩图书列表

针对图书列表⻚有两处变化

1. 拦截器进行了强制登录校验,如果校验失败,则http状态码返回401,此时会⾛ajax的error逻辑处理
2. 接⼝返回结果发生了变化

图书列表代码修改：

function getBookList() {$.ajax({type: "get",url: "/book/getListByPage"+location.search,success: function (result) {console.log(result);if (result == null || result.data == null) {location.href = "login.html";return;}if (result != null) {var finalHtml = "";var data = result.data;for (var book of data.records) {finalHtml += '<tr>';finalHtml += '<td><input type="checkbox" name="selectBook" value="'+book.id+'" id="selectBook" class="book-select"></td>'finalHtml += '<td>' + book.id + '</td>';finalHtml += '<td>' + book.bookName + '</td>';finalHtml += '<td>' + book.author + '</td>';finalHtml += '<td>' + book.count + '</td>';finalHtml += '<td>' + book.price + '</td>';finalHtml += '<td>' + book.publish + '</td>';finalHtml += '<td>' + book.statusCN + '</td>';finalHtml += '<td><div class="op">';finalHtml += '<a href="book_update.html?bookId=' + book.id + '">修改</a>'finalHtml += '<a href="javascript:void(0)" onclick="deleteBook(' + book.id + ')">删除</a>'finalHtml += '</div></td>';finalHtml += "</tr>";}$("tbody").html(finalHtml);//翻页信息$("#pageContainer").jqPaginator({totalCounts: data.total, //总记录数pageSize: 10,    //每页的个数visiblePages: 5, //可视页数currentPage: data.pageRequest.currentPage,  //当前页码first: '<li class="page-item"><a class="page-link">首页</a></li>',prev: '<li class="page-item"><a class="page-link" href="javascript:void(0);">上一页<\/a><\/li>',next: '<li class="page-item"><a class="page-link" href="javascript:void(0);">下一页<\/a><\/li>',last: '<li class="page-item"><a class="page-link" href="javascript:void(0);">最后一页<\/a><\/li>',page: '<li class="page-item"><a class="page-link" href="javascript:void(0);">{{page}}<\/a><\/li>',//页面初始化和页码点击时都会执行onPageChange: function (page, type) {console.log("第" + page + "⻚, 类型:" + type);if (type != 'init') {location.href = "book_list.html?currentPage=" + page;}}});}},error(error) {if(error.status == 401) {location.href = "login.html";}}});
}

🚩删除图书

function deleteBook(id) {//...代码省略success: function (result) {if(result.status=="SUCCESS" || result.data==""){//重新刷新⻚⾯location.href = "book_list.html"}else{alert(result.data);}},error: function (error) {if (error != null && error.status == 401) {//⽤⼾未登录location.href = "login.html";}}//...代码省略
}

🚩批量删除图书

function batchDelete() {var isDelete = confirm("确认批量删除?");if (isDelete) {//获取复选框的idvar ids = [];$("input:checkbox[name='selectBook']:checked").each(function () {ids.push($(this).val());});console.log(ids);//批量删除$.ajax({type: "post",url: "/book/batchDeleteBook?ids="+ids,success: function (result) {if (result.status=="SUCCESS" || result.data==true) {alert("删除成功");//重新刷新⻚⾯location.href = "book_list.html"}},error: function (error) {if (error != null && error.status == 401) {//⽤⼾未登录location.href = "login.html";}}});}
}

🚩添加图书

function add() {$.ajax({type: "post",url: "/book/addBook",data: $("#addBook").serialize(),success: function (result) {console.log(result);console.log(result.data);if (result.status == "SUCCESS" && result.data == "") {location.href = "book_list.html"} else {console.log(result);alert("添加失败:" + result.data);}},error: function (error) {if (error != null && error.status == 401) {//⽤⼾未登录alert("⽤⼾未登录");location.href = "login.html";}}});
}

🚩获取图书详情

$.ajax({type:"get",url: "/book/queryBookById"+location.search,success:function(book){if (result.status == "SUCCESS" && result.data != null) {var book = result.data;if (book != null) {$("#bookId").val(book.id);$("#bookName").val(book.bookName);$("#bookAuthor").val(book.author);$("#bookStock").val(book.count);$("#bookPrice").val(book.price);$("#bookPublisher").val(book.publish);$("#bookStatus").val(book.status);}}},error: function (error) {if (error != null && error.status == 401) {//⽤⼾未登录alert("⽤⼾未登录");location.href = "login.html";}}
});

🚩修改图书

function update() {$.ajax({type: "post",url: "/book/updateBook",data: $("#updateBook").serialize(),success: function (result) {if (result.status == "SUCCESS" || result.data == "") {location.href = "book_list.html"} else {console.log(result);alert("修改失败:" + result.data);}},error: function (error) {if (error != null && error.status == 401) {//⽤⼾未登录location.href = "login.html";}}});
}

⭕总结

关于《【JavaEE进阶】 图书管理系统开发日记——捌》就讲解到这儿，图书管理系统到此也就开发完毕了，感谢大家的支持，欢迎各位留言交流以及批评指正，如果文章对您有帮助或者觉得作者写的还不错可以点一下关注，点赞，收藏支持一下！