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源码分析 | Mybatis接口没有实现类为什么可以执行增删改查


作者:小傅哥

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一、前言介绍

MyBatis 是一款非常优秀的持久层框架,相对于IBatis更是精进了不少。与此同时它还提供了很多的扩展点,比如最常用的插件;语言驱动器,执行器,对象工厂,对象包装器工厂等等都可以扩展。那么,如果想成为一个有深度的男人(程序猿),还是应该好好的学习一下这款开源框架的源码,以此可以更好的领会设计模式的精髓(面试?)。其实可能平常的业务开发中,并不会去深究各个框架的源代码,也常常会听到即使不会也可以开发代码。但!每个人的目标不同,就像;代码写的好工资加的少(没有bug怎么看出你工作嘞!),好!为了改变世界,开始分析喽!

在分析之前先出一个题,看看你适合看源码不;

@Test
public void test(){
B b = new B();
b.scan(); //我的输出结果是什么?
}
static class A {
public void scan(){
doScan();
}
protected void doScan(){
System.out.println("A.doScan");
}
}
static class B extends A {
@Override
protected void doScan() {
System.out.println("B.doScan");
}
}

其实无论你的答案对错,都不影响你对源码的分析。只不过,往往在一些框架中会有很多的设计模式和开发技巧,如果上面的代码在你平时的开发中几乎没用过,那么可能你暂时更多的还是开发着CRUD的功能(莫慌,我还写过PHP呢)。

接下来先分析Mybatis单独使用时的源码执行过程,再分析Mybatis+Spring整合源码,好!开始。

二、案例工程

为了更好的分析,我们创建一个Mybaits的案例工程,其中包括;Mybatis单独使用、Mybatis+Spring整合使用

itstack-demo-mybatis
└── src
├── main
│ ├── java
│ │ └── org.itstack.demo
│ │ ├── dao
│ │ │├── ISchool.java
│ │ │└── IUserDao.java
│ │ └── interfaces
│ │ ├── School.java
│ │ └── User.java
│ ├── resources
│ │ ├── mapper
│ │ │ ├── School_Mapper.xml
│ │ │ └── User_Mapper.xml
│ │ ├── props
│ │ │ └── jdbc.properties
│ │ ├── spring
│ │ │ ├── mybatis-config-datasource.xml
│ │ │ └── spring-config-datasource.xml
│ │ ├── logback.xml
│ │ ├── mybatis-config.xml
│ │ └── spring-config.xml
│ └── webapp
│ └── WEB-INF
└── test
└── java
└── org.itstack.demo.test
├── MybatisApiTest.java
└── SpringApiTest.java

三、环境配置

JDK1.8IDEA 2019.3.1mybatis 3.4.6 {不同版本源码略有差异和bug修复}mybatis-spring 1.3.2 {以下源码分析会说代码行号,注意不同版本可能会有差异}

四、(mybatis)源码分析


org.mybatis
mybatis
3.4.6

Mybatis的整个源码还是很大的,以下主要将部分核心内容进行整理分析,以便于后续分析Mybatis与Spring整合的源码部分。简要包括;容器初始化、配置文件解析、Mapper加载与动态代理。

1. 从一个简单的案例开始

要学习Mybatis源码,最好的方式一定是从一个简单的点进入,而不是从Spring整合开始分析。SqlSessionFactory是整个Mybatis的核心实例对象,SqlSessionFactory对象的实例又通过SqlSessionFactoryBuilder对象来获得。SqlSessionFactoryBuilder对象可以从XML配置文件加载配置信息,然后创建SqlSessionFactory。如下例子:

>MybatisApiTest.java

public class MybatisApiTest {

@Test
public void test_queryUserInfoById() {
String resource = "spring/mybatis-config-datasource.xml";
Reader reader;
try {
reader = Resources.getResourceAsReader(resource);
SqlSessionFactory sqlMapper = new SqlSessionFactoryBuilder().build(reader);

SqlSession session = sqlMapper.openSession();
try {
User user = session.selectOne("org.itstack.demo.dao.IUserDao.queryUserInfoById", 1L);
System.out.println(JSON.toJSONString(user));
} finally {
session.close();
reader.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}

}

>dao/IUserDao.java

public interface IUserDao {

User queryUserInfoById(Long id);

}

>spring/mybatis-config-datasource.xml

如果一切顺利,那么会有如下结果:

{"age":18,"createTime":1571376957000,"id":1,"name":"花花","updateTime":1571376957000}

从上面的代码块可以看到,核心代码;SqlSessionFactoryBuilder().build(reader),负责Mybatis配置文件的加载、解析、构建等职责,直到最终可以通过SqlSession来执行并返回结果。

2. 容器初始化

从上面代码可以看到,SqlSessionFactory是通过SqlSessionFactoryBuilder工厂类创建的,而不是直接使用构造器。容器的配置文件加载和初始化流程如下:

源码分析 | Mybatis接口没有实现类为什么可以执行增删改查 1

流程核心类

– SqlSessionFactoryBuilder

– XMLConfigBuilder

– XPathParser

– Configuration

>SqlSessionFactoryBuilder.java

public class SqlSessionFactoryBuilder {

public SqlSessionFactory build(Reader reader) {
return build(reader, null, null);
}

public SqlSessionFactory build(Reader reader, String environment) {
return build(reader, environment, null);
}

public SqlSessionFactory build(Reader reader, Properties properties) {
return build(reader, null, properties);
}

public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}

public SqlSessionFactory build(InputStream inputStream) {
return build(inputStream, null, null);
}

public SqlSessionFactory build(InputStream inputStream, String environment) {
return build(inputStream, environment, null);
}

public SqlSessionFactory build(InputStream inputStream, Properties properties) {
return build(inputStream, null, properties);
}

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
inputStream.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}

public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}

}

从上面的源码可以看到,SqlSessionFactory提供三种方式build构建对象;

字节流:java.io.InputStream字符流:java.io.Reader配置类:org.apache.ibatis.session.Configuration

那么,字节流、字符流都会创建配置文件解析类:XMLConfigBuilder,并通过parser.parse()生成Configuration,最后调用配置类构建方法生成SqlSessionFactory。

>XMLConfigBuilder.java

public class XMLConfigBuilder extends BaseBuilder {

private boolean parsed;
private final XPathParser parser;
private String environment;
private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();

...
public XMLConfigBuilder(Reader reader, String environment, Properties props) {
this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
}
...
}

XMLConfigBuilder对于XML文件的加载和解析都委托于XPathParser,最终使用JDK自带的javax.xml进行XML解析(XPath)XPathParser(Reader reader, boolean validation, Properties variables, EntityResolver entityResolver)

1. reader:使用字符流创建新的输入源,用于对XML文件的读取

2. validation:是否进行DTD校验

3. variables:属性配置信息

4. entityResolver:Mybatis硬编码了new XMLMapperEntityResolver()提供XML默认解析器

>XMLMapperEntityResolver.java

public class XMLMapperEntityResolver implements EntityResolver {

private static final String IBATIS_CONFIG_SYSTEM = "ibatis-3-config.dtd";
private static final String IBATIS_MAPPER_SYSTEM = "ibatis-3-mapper.dtd";
private static final String MYBATIS_CONFIG_SYSTEM = "mybatis-3-config.dtd";
private static final String MYBATIS_MAPPER_SYSTEM = "mybatis-3-mapper.dtd";

private static final String MYBATIS_CONFIG_DTD = "org/apache/ibatis/builder/xml/mybatis-3-config.dtd";
private static final String MYBATIS_MAPPER_DTD = "org/apache/ibatis/builder/xml/mybatis-3-mapper.dtd";

/*
* Converts a public DTD into a local one
*
* @param publicId The public id that is what comes after "PUBLIC"
* @param systemId The system id that is what comes after the public id.
* @return The InputSource for the DTD
*
* @throws org.xml.sax.SAXException If anything goes wrong
*/
@Override
public InputSource resolveEntity(String publicId, String systemId) throws SAXException {
try {
if (systemId != null) {
String lowerCaseSystemId = systemId.toLowerCase(Locale.ENGLISH);
if (lowerCaseSystemId.contains(MYBATIS_CONFIG_SYSTEM) || lowerCaseSystemId.contains(IBATIS_CONFIG_SYSTEM)) {
return getInputSource(MYBATIS_CONFIG_DTD, publicId, systemId);
} else if (lowerCaseSystemId.contains(MYBATIS_MAPPER_SYSTEM) || lowerCaseSystemId.contains(IBATIS_MAPPER_SYSTEM)) {
return getInputSource(MYBATIS_MAPPER_DTD, publicId, systemId);
}
}
return null;
} catch (Exception e) {
throw new SAXException(e.toString());
}
}

private InputSource getInputSource(String path, String publicId, String systemId) {
InputSource source = null;
if (path != null) {
try {
InputStream in = Resources.getResourceAsStream(path);
source = new InputSource(in);
source.setPublicId(publicId);
source.setSystemId(systemId);
} catch (IOException e) {
// ignore, null is ok
}
}
return source;
}

}

Mybatis依赖于dtd文件进行进行解析,其中的ibatis-3-config.dtd主要是用于兼容用途getInputSource(String path, String publicId, String systemId)的调用里面有两个参数publicId(公共标识符)和systemId(系统标示符)

>XPathParser.java

public XPathParser(Reader reader, boolean validation, Properties variables, EntityResolver entityResolver) {
commonConstructor(validation, variables, entityResolver);
this.document = createDocument(new InputSource(reader));
}

private void commonConstructor(boolean validation, Properties variables, EntityResolver entityResolver) {
this.validation = validation;
this.entityResolver = entityResolver;
this.variables = variables;
XPathFactory factory = XPathFactory.newInstance();
this.xpath = factory.newXPath();
}

private Document createDocument(InputSource inputSource) {
// important: this must only be called AFTER common constructor
try {
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setValidating(validation);
factory.setNamespaceAware(false);
factory.setIgnoringComments(true);
factory.setIgnoringElementContentWhitespace(false);
factory.setCoalescing(false);
factory.setExpandEntityReferences(true);
DocumentBuilder builder = factory.newDocumentBuilder();
builder.setEntityResolver(entityResolver);
builder.setErrorHandler(new ErrorHandler() {
@Override
public void error(SAXParseException exception) throws SAXException {
throw exception;
}
@Override
public void fatalError(SAXParseException exception) throws SAXException {
throw exception;
}
@Override
public void warning(SAXParseException exception) throws SAXException {
}
});
return builder.parse(inputSource);
} catch (Exception e) {
throw new BuilderException("Error creating document instance. Cause: " + e, e);
}

}

从上到下可以看到主要是为了创建一个Mybatis的文档解析器,最后根据builder.parse(inputSource)返回Document得到XPathParser实例后,接下来在调用方法:this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);

XMLConfigBuilder.this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);

private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
super(new Configuration());
ErrorContext.instance().resource("SQL Mapper Configuration");
this.configuration.setVariables(props);
this.parsed = false;
this.environment = environment;
this.parser = parser;
}

其中调用了父类的构造函数

public abstract class BaseBuilder {
protected final Configuration configuration;
protected final TypeAliasRegistry typeAliasRegistry;
protected final TypeHandlerRegistry typeHandlerRegistry;

public BaseBuilder(Configuration configuration) {
this.configuration = configuration;
this.typeAliasRegistry = this.configuration.getTypeAliasRegistry();
this.typeHandlerRegistry = this.configuration.getTypeHandlerRegistry();
}
}

XMLConfigBuilder创建完成后,sqlSessionFactoryBuild调用parser.parse()创建Configuration

public class XMLConfigBuilder extends BaseBuilder {
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
}

3. 配置文件解析

这一部分是整个XML文件解析和装载的核心内容,其中包括;

属性解析propertiesElement加载settings节点settingsAsProperties载自定义VFS loadCustomVfs解析类型别名typeAliasesElement加载插件pluginElement加载对象工厂objectFactoryElement创建对象包装器工厂objectWrapperFactoryElement加载反射工厂reflectorFactoryElement元素设置settingsElement加载环境配置environmentsElement数据库厂商标识加载databaseIdProviderElement加载类型处理器typeHandlerElement(核心)加载mapper文件mapperElement

parseConfiguration(parser.evalNode("/configuration"));

private void parseConfiguration(XNode root) {
try {
//issue #117 read properties first
//属性解析propertiesElement
propertiesElement(root.evalNode("properties"));
//加载settings节点settingsAsProperties
Properties settings = settingsAsProperties(root.evalNode("settings"));
//加载自定义VFS loadCustomVfs
loadCustomVfs(settings);
//解析类型别名typeAliasesElement
typeAliasesElement(root.evalNode("typeAliases"));
//加载插件pluginElement
pluginElement(root.evalNode("plugins"));
//加载对象工厂objectFactoryElement
objectFactoryElement(root.evalNode("objectFactory"));
//创建对象包装器工厂objectWrapperFactoryElement
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
//加载反射工厂reflectorFactoryElement
reflectorFactoryElement(root.evalNode("reflectorFactory"));
//元素设置
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
//加载环境配置environmentsElement
environmentsElement(root.evalNode("environments"));
//数据库厂商标识加载databaseIdProviderElement
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
//加载类型处理器typeHandlerElement
typeHandlerElement(root.evalNode("typeHandlers"));
//加载mapper文件mapperElement
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}

所有的root.evalNode()底层都是调用XML DOM方法:Object evaluate(String expression, Object item, QName returnType),表达式参数expression,通过XObject resultObject = eval( expression, item )返回最终节点内容,可以参考http://mybatis.org/dtd/mybatis-3-config.dtd,如下;

















mybatis-3-config.dtd 定义文件中有11个配置文件,如下;

properties?, settings?, typeAliases?, typeHandlers?, objectFactory?, objectWrapperFactory?, reflectorFactory?, plugins?, environments?, databaseIdProvider?, mappers?

以上每个配置都是可选。最终配置内容会保存到org.apache.ibatis.session.Configuration,如下;

public class Configuration {

protected Environment environment;
// 允许在嵌套语句中使用分页(RowBounds)。如果允许使用则设置为false。默认为false
protected boolean safeRowBoundsEnabled;
// 允许在嵌套语句中使用分页(ResultHandler)。如果允许使用则设置为false。
protected boolean safeResultHandlerEnabled = true;
// 是否开启自动驼峰命名规则(camel case)映射,即从经典数据库列名 A_COLUMN 到经典 Java 属性名 aColumn 的类似映射。默认false
protected boolean mapUnderscoreToCamelCase;
// 当开启时,任何方法的调用都会加载该对象的所有属性。否则,每个属性会按需加载。默认值false (true in ≤3.4.1)
protected boolean aggressiveLazyLoading;
// 是否允许单一语句返回多结果集(需要兼容驱动)。
protected boolean multipleResultSetsEnabled = true;
// 允许 JDBC 支持自动生成主键,需要驱动兼容。这就是insert时获取mysql自增主键/oracle sequence的开关。注:一般来说,这是希望的结果,应该默认值为true比较合适。
protected boolean useGeneratedKeys;
// 使用列标签代替列名,一般来说,这是希望的结果
protected boolean useColumnLabel = true;
// 是否启用缓存 {默认是开启的,可能这也是你的面试题}
protected boolean cacheEnabled = true;
// 指定当结果集中值为 null 的时候是否调用映射对象的 setter(map 对象时为 put)方法,这对于有 Map.keySet() 依赖或 null 值初始化的时候是有用的。
protected boolean callSettersOnNulls;
// 允许使用方法签名中的名称作为语句参数名称。 为了使用该特性,你的工程必须采用Java 8编译,并且加上-parameters选项。(从3.4.1开始)
protected boolean useActualParamName = true;
//当返回行的所有列都是空时,MyBatis默认返回null。 当开启这个设置时,MyBatis会返回一个空实例。 请注意,它也适用于嵌套的结果集 (i.e. collectioin and association)。(从3.4.2开始) 注:这里应该拆分为两个参数比较合适, 一个用于结果集,一个用于单记录。通常来说,我们会希望结果集不是null,单记录仍然是null
protected boolean returnInstanceForEmptyRow;
// 指定 MyBatis 增加到日志名称的前缀。
protected String logPrefix;
// 指定 MyBatis 所用日志的具体实现,未指定时将自动查找。一般建议指定为slf4j或log4j
protected Class logImpl;
// 指定VFS的实现, VFS是mybatis提供的用于访问AS内资源的一个简便接口
protected Class vfsImpl;
// MyBatis 利用本地缓存机制(Local Cache)防止循环引用(circular references)和加速重复嵌套查询。 默认值为 SESSION,这种情况下会缓存一个会话中执行的所有查询。 若设置值为 STATEMENT,本地会话仅用在语句执行上,对相同 SqlSession 的不同调用将不会共享数据。
protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
// 当没有为参数提供特定的 JDBC 类型时,为空值指定 JDBC 类型。 某些驱动需要指定列的 JDBC 类型,多数情况直接用一般类型即可,比如 NULL、VARCHAR 或 OTHER。
protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
// 指定对象的哪个方法触发一次延迟加载。
protected Set lazyLoadTriggerMethods = new HashSet(Arrays.asList(new String[] { "equals", "clone", "hashCode", "toString" }));
// 设置超时时间,它决定驱动等待数据库响应的秒数。默认不超时
protected Integer defaultStatementTimeout;
// 为驱动的结果集设置默认获取数量。
protected Integer defaultFetchSize;
// SIMPLE 就是普通的执行器;REUSE 执行器会重用预处理语句(prepared statements); BATCH 执行器将重用语句并执行批量更新。
protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
// 指定 MyBatis 应如何自动映射列到字段或属性。 NONE 表示取消自动映射;PARTIAL 只会自动映射没有定义嵌套结果集映射的结果集。 FULL 会自动映射任意复杂的结果集(无论是否嵌套)。
protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;
// 指定发现自动映射目标未知列(或者未知属性类型)的行为。这个值应该设置为WARNING比较合适
protected AutoMappingUnknownColumnBehavior autoMappingUnknownColumnBehavior = AutoMappingUnknownColumnBehavior.NONE;
// settings下的properties属性
protected Properties variables = new Properties();
// 默认的反射器工厂,用于操作属性、构造器方便
protected ReflectorFactory reflectorFactory = new DefaultReflectorFactory();
// 对象工厂, 所有的类resultMap类都需要依赖于对象工厂来实例化
protected ObjectFactory objectFactory = new DefaultObjectFactory();
// 对象包装器工厂,主要用来在创建非原生对象,比如增加了某些监控或者特殊属性的代理类
protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();
// 延迟加载的全局开关。当开启时,所有关联对象都会延迟加载。特定关联关系中可通过设置fetchType属性来覆盖该项的开关状态。
protected boolean lazyLoadingEnabled = false;
// 指定 Mybatis 创建具有延迟加载能力的对象所用到的代理工具。MyBatis 3.3+使用JAVASSIST
protected ProxyFactory proxyFactory = new JavassistProxyFactory(); // #224 Using internal Javassist instead of OGNL
// MyBatis 可以根据不同的数据库厂商执行不同的语句,这种多厂商的支持是基于映射语句中的 databaseId 属性。
protected String databaseId;
...
}

以上可以看到,Mybatis把所有的配置;resultMap、Sql语句、插件、缓存等都维护在Configuration中。这里还有一个小技巧,在Configuration还有一个StrictMap内部类,它继承于HashMap完善了put时防重、get时取不到值的异常处理,如下;

protected static class StrictMap extends HashMap {

private static final long serialVersionUID = -4950446264854982944L;
private final String name;

public StrictMap(String name, int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor);
this.name = name;
}

public StrictMap(String name, int initialCapacity) {
super(initialCapacity);
this.name = name;
}

public StrictMap(String name) {
super();
this.name = name;
}

public StrictMap(String name, Map m) {
super(m);
this.name = name;
}
}

(核心)加载mapper文件mapperElement

Mapper文件处理是Mybatis框架的核心服务,所有的SQL语句都编写在Mapper中,这块也是我们分析的重点,其他模块可以后续讲解。

>XMLConfigBuilder.parseConfiguration()->mapperElement(root.evalNode(“mappers”));

private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
// 如果要同时使用package自动扫描和通过mapper明确指定要加载的mapper,一定要确保package自动扫描的范围不包含明确指定的mapper,否则在通过package扫描的interface的时候,尝试加载对应xml文件的loadXmlResource()的逻辑中出现判重出错,报org.apache.ibatis.binding.BindingException异常,即使xml文件中包含的内容和mapper接口中包含的语句不重复也会出错,包括加载mapper接口时自动加载的xml mapper也一样会出错。
if ("package".equals(child.getName())) {
String mapperPackage = child.getStringAttribute("name");
configuration.addMappers(mapperPackage);
} else {
String resource = child.getStringAttribute("resource");
String url = child.getStringAttribute("url");
String mapperClass = child.getStringAttribute("class");
if (resource != null && url == null && mapperClass == null) {
ErrorContext.instance().resource(resource);
InputStream inputStream = Resources.getResourceAsStream(resource);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url != null && mapperClass == null) {
ErrorContext.instance().resource(url);
InputStream inputStream = Resources.getUrlAsStream(url);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url == null && mapperClass != null) {
Class mapperInterface = Resources.classForName(mapperClass);
configuration.addMapper(mapperInterface);
} else {
throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
}
}
}
}
}

Mybatis提供了两类配置Mapper的方法,第一类是使用package自动搜索的模式,这样指定package下所有接口都会被注册为mapper,也是在Spring中比较常用的方式,例如:

另外一类是明确指定Mapper,这又可以通过resource、url或者class进行细分,例如;

4. Mapper加载与动态代理

通过package方式自动搜索加载,生成对应的mapper代理类,代码块和流程,如下;

private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
if ("package".equals(child.getName())) {
String mapperPackage = child.getStringAttribute("name");
configuration.addMappers(mapperPackage);
} else {
...
}
}
}
}

源码分析 | Mybatis接口没有实现类为什么可以执行增删改查 2

Mapper加载到生成代理对象的流程中,主要的核心类包括;

XMLConfigBuilderConfigurationMapperRegistryMapperAnnotationBuilderMapperProxyFactory

>MapperRegistry.java

解析加载Mapper

public void addMappers(String packageName, Class superType) {
// mybatis框架提供的搜索classpath下指定package以及子package中符合条件(注解或者继承于某个类/接口)的类,默认使用Thread.currentThread().getContextClassLoader()返回的加载器,和spring的工具类殊途同归。
ResolverUtil resolverUtil = new ResolverUtil();
// 无条件的加载所有的类,因为调用方传递了Object.class作为父类,这也给以后的指定mapper接口预留了余地
resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
// 所有匹配的calss都被存储在ResolverUtil.matches字段中
Set> mapperSet = resolverUtil.getClasses();
for (Class mapperClass : mapperSet) {
//调用addMapper方法进行具体的mapper类/接口解析
addMapper(mapperClass);
}
}

生成代理类:MapperProxyFactory

public void addMapper(Class type) {
// 对于mybatis mapper接口文件,必须是interface,不能是class
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
// 为mapper接口创建一个MapperProxyFactory代理
knownMappers.put(type, new MapperProxyFactory(type));
// It's important that the type is added before the parser is run
// otherwise the binding may automatically be attempted by the
// mapper parser. If the type is already known, it won't try.
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}

在MapperRegistry中维护了接口类与代理工程的映射关系,knownMappers;

private final Map> knownMappers = new HashMap>();

>MapperProxyFactory.java

public class MapperProxyFactory {
private final Class mapperInterface;
private final Map methodCache = new ConcurrentHashMap();
public MapperProxyFactory(Class mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class getMapperInterface() {
return mapperInterface;
}
public Map getMethodCache() {
return methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}

如上是Mapper的代理类工程,构造函数中的mapperInterface就是对应的接口类,当实例化时候会获得具体的MapperProxy代理,里面主要包含了SqlSession。

五、(mybatis-spring)源码分析


org.mybatis
mybatis-spring
1.3.2

作为一款好用的ORM框架,一定是萝莉脸(单纯)、御姐心(强大),铺的了床(屏蔽与JDBC直接打交道)、暖的了房(速度性能好)!鉴于这些优点几乎在国内互联网大部分开发框架都会使用到Mybatis,尤其在一些需要高性能的场景下需要优化sql那么一定需要手写sql在xml中。那么,准备好了吗!开始分析分析它的源码;

1. 从一个简单的案例开始

与分析mybatis源码一样,先做一个简单的案例;定义dao、编写配置文件、junit单元测试;

>SpringApiTest.java

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration("classpath:spring-config.xml")
public class SpringApiTest {

private Logger logger = LoggerFactory.getLogger(SpringApiTest.class);

@Resource
private ISchoolDao schoolDao;
@Resource
private IUserDao userDao;

@Test
public void test_queryRuleTreeByTreeId(){
School ruleTree = schoolDao.querySchoolInfoById(1L);
logger.info(JSON.toJSONString(ruleTree));

User user = userDao.queryUserInfoById(1L);
logger.info(JSON.toJSONString(user));
}

}

>spring-config-datasource.xml

如果一切顺利,那么会有如下结果:

{"address":"北京市海淀区颐和园路5号","createTime":1571376957000,"id":1,"name":"北京大学","updateTime":1571376957000}
{"age":18,"createTime":1571376957000,"id":1,"name":"花花","updateTime":1571376957000}

从上面单元测试的代码可以看到,两个没有方法体的注解就这么神奇的执行了我们的xml中的配置语句并输出了结果。其实主要得益于以下两个类;

org.mybatis.spring.SqlSessionFactoryBeanorg.mybatis.spring.mapper.MapperScannerConfigurer

2. 扫描装配注册(MapperScannerConfigurer)

MapperScannerConfigurer为整个Dao接口层生成动态代理类注册,启动到了核心作用。这个类实现了如下接口,用来对扫描的Mapper进行处理:

BeanDefinitionRegistryPostProcessorInitializingBeanApplicationContextAwareBeanNameAware

整体类图如下;

源码分析 | Mybatis接口没有实现类为什么可以执行增删改查 3

执行流程如下;

源码分析 | Mybatis接口没有实现类为什么可以执行增删改查 4

上面的类图+流程图,其实已经很清楚的描述了MapperScannerConfigurer初始化过程,但对于头一次看的新人来说依旧是我太难了,好继续!

>MapperScannerConfigurer.java & 部分截取

@Override
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
if (this.processPropertyPlaceHolders) {
processPropertyPlaceHolders();
}
ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
scanner.setAddToConfig(this.addToConfig);
scanner.setAnnotationClass(this.annotationClass);
scanner.setMarkerInterface(this.markerInterface);
scanner.setSqlSessionFactory(this.sqlSessionFactory);
scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
scanner.setResourceLoader(this.applicationContext);
scanner.setBeanNameGenerator(this.nameGenerator);
scanner.registerFilters();
scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
}

实现了BeanDefinitionRegistryPostProcessor.postProcessBeanDefinitionRegistry用于注册Bean到Spring容器中306行:new ClassPathMapperScanner(registry); 硬编码类路径扫描器,用于解析Mybatis的Mapper文件317行:scanner.scan 对Mapper进行扫描。这里包含了一个继承类实现关系的调用,也就是本文开头的测试题。

>ClassPathMapperScanner.java & 部分截取

@Override
public Set doScan(String... basePackages) {
Set beanDefinitions = super.doScan(basePackages);
if (beanDefinitions.isEmpty()) {
logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
} else {
processBeanDefinitions(beanDefinitions);
}
return beanDefinitions;
}

优先调用父类的super.doScan(basePackages);进行注册Bean信息

>ClassPathBeanDefinitionScanner.java & 部分截取

protected Set doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set beanDefinitions = new LinkedHashSet();
for (String basePackage : basePackages) {
Set candidates = findCandidateComponents(basePackage);
for (BeanDefinition candidate : candidates) {
ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
candidate.setScope(scopeMetadata.getScopeName());
String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
if (candidate instanceof AbstractBeanDefinition) {
postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
}
if (candidate instanceof AnnotatedBeanDefinition) {
AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate)
}
if (checkCandidate(beanName, candidate)) {
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
definitionHolder =
AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.regi
beanDefinitions.add(definitionHolder);
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}

优先调用了父类的doScan方法,用于Mapper扫描和Bean的定义以及注册到DefaultListableBeanFactory。{DefaultListableBeanFactory是Spring中IOC容器的始祖,所有需要实例化的类都需要注册进来,之后在初始化}272行:findCandidateComponents(basePackage),扫描package包路径,对于注解类的有另外的方式,大同小异288行:registerBeanDefinition(definitionHolder, this.registry);注册Bean信息的过程,最终会调用到:org.springframework.beans.factory.support.DefaultListableBeanFactory

>ClassPathMapperScanner.java & 部分截取

**processBeanDefinitions(beanDefinitions);**

private void processBeanDefinitions(Set beanDefinitions) {
GenericBeanDefinition definition;
for (BeanDefinitionHolder holder : beanDefinitions) {
definition = (GenericBeanDefinition) holder.getBeanDefinition();
if (logger.isDebugEnabled()) {
logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName()
+ "' and '" + definition.getBeanClassName() + "' mapperInterface");
}
// the mapper interface is the original class of the bean
// but, the actual class of the bean is MapperFactoryBean
definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
definition.setBeanClass(this.mapperFactoryBean.getClass());
definition.getPropertyValues().add("addToConfig", this.addToConfig);
boolean explicitFactoryUsed = false;
if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) {
definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
explicitFactoryUsed = true;
} else if (this.sqlSessionFactory != null) {
definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
explicitFactoryUsed = true;
}
if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
if (explicitFactoryUsed) {
logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
}
definition.getPropertyValues().add("sqlSessionTemplate", new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
explicitFactoryUsed = true;
} else if (this.sqlSessionTemplate != null) {
if (explicitFactoryUsed) {
logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
}
definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
explicitFactoryUsed = true;
}
if (!explicitFactoryUsed) {
if (logger.isDebugEnabled()) {
logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
}
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
}
}
}

163行:super.doScan(basePackages);,调用完父类方法后开始执行内部方法:processBeanDefinitions(beanDefinitions)186行:definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); 设置BeanName参数,也就是我们的:ISchoolDao、IUserDao187行:definition.setBeanClass(this.mapperFactoryBean.getClass());,设置BeanClass,接口本身是没有类的,那么这里将MapperFactoryBean类设置进来,最终所有的dao层接口类都是这个MapperFactoryBean

>MapperFactoryBean.java & 部分截取

这个类有继承也有接口实现,最好先了解下整体类图,如下;

源码分析 | Mybatis接口没有实现类为什么可以执行增删改查 5

这个类就非常重要了,最终所有的sql信息执行都会通过这个类获取getObject(),也就是SqlSession获取mapper的代理类:MapperProxyFactory->MapperProxy

public class MapperFactoryBean extends SqlSessionDaoSupport implements FactoryBean {

private Class mapperInterface;

private boolean addToConfig = true;

public MapperFactoryBean() {
//intentionally empty
}

public MapperFactoryBean(Class mapperInterface) {
this.mapperInterface = mapperInterface;
}

/**
* 当SpringBean容器初始化时候会调用到checkDaoConfig(),他是继承类中的抽象方法
* {@inheritDoc}
*/
@Override
protected void checkDaoConfig() {
super.checkDaoConfig();

notNull(this.mapperInterface, "Property 'mapperInterface' is required");

Configuration configuration = getSqlSession().getConfiguration();
if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
try {
configuration.addMapper(this.mapperInterface);
} catch (Exception e) {
logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", e);
throw new IllegalArgumentException(e);
} finally {
ErrorContext.instance().reset();
}
}
}

/**
* {@inheritDoc}
*/
@Override
public T getObject() throws Exception {
return getSqlSession().getMapper(this.mapperInterface);
}

...
}

72行:checkDaoConfig(),当SpringBean容器初始化时候会调用到checkDaoConfig(),他是继承类中的抽象方法95行:getSqlSession().getMapper(this.mapperInterface);,通过接口获取Mapper(代理类),调用过程如下;

- DefaultSqlSession.getMapper(Class type),获取Mapper

- Configuration.getMapper(Class type, SqlSession sqlSession),从配置中获取

- MapperRegistry.getMapper(Class type, SqlSession sqlSession),从注册中心获取到实例化生成

public T getMapper(Class type, SqlSession sqlSession) {
final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}

- mapperProxyFactory.newInstance(sqlSession);,通过反射工程生成MapperProxy

@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}

>MapperProxy.java & 部分截取

public class MapperProxy implements InvocationHandler, Serializable {

private static final long serialVersionUID = -6424540398559729838L;
private final SqlSession sqlSession;
private final Class mapperInterface;
private final Map methodCache;

public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}

private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}

@UsesJava7
private Object invokeDefaultMethod(Object proxy, Method method, Object[] args)
throws Throwable {
final Constructor constructor = MethodHandles.Lookup.class
.getDeclaredConstructor(Class.class, int.class);
if (!constructor.isAccessible()) {
constructor.setAccessible(true);
}
final Class declaringClass = method.getDeclaringClass();
return constructor
.newInstance(declaringClass,
MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED
| MethodHandles.Lookup.PACKAGE | MethodHandles.Lookup.PUBLIC)
.unreflectSpecial(method, declaringClass).bindTo(proxy).invokeWithArguments(args);
}

...
}

58行:final MapperMethod mapperMethod = cachedMapperMethod(method);,从缓存中获取MapperMethod59行:mapperMethod.execute(sqlSession, args);,执行SQL语句,并返回结果(到这关于查询获取结果就到骨头(干)层了);INSERT、UPDATE、DELETE、SELECT

public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}

以上对于MapperScannerConfigurer这一层就分析完了,从扫描定义注入到为Spring容器准备Bean的信息,代理、反射、SQL执行,基本就包括全部核心内容了,接下来在分析下SqlSessionFactoryBean

3. SqlSession容器工厂初始化(SqlSessionFactoryBean)

SqlSessionFactoryBean初始化过程中需要对一些自身内容进行处理,因此也需要实现如下接口;

FactoryBeanInitializingBean -> void afterPropertiesSet() throws ExceptionApplicationListener

源码分析 | Mybatis接口没有实现类为什么可以执行增删改查 6

以上的流程其实已经很清晰的描述整个核心流程,但同样对于新手上路会有障碍,那么!好,继续!

>SqlSessionFactoryBean.java & 部分截取

public void afterPropertiesSet() throws Exception {
notNull(dataSource, "Property 'dataSource' is required");
notNull(sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");
state((configuration == null && configLocation == null) || !(configuration != null && configLocation != null),
"Property 'configuration' and 'configLocation' can not specified with together");
this.sqlSessionFactory = buildSqlSessionFactory();
}

afterPropertiesSet(),InitializingBean接口为bean提供了初始化方法的方式,它只包括afterPropertiesSet方法,凡是继承该接口的类,在初始化bean的时候都会执行该方法。380行:buildSqlSessionFactory();内部方法构建,核心功能继续往下看。

>SqlSessionFactoryBean.java & 部分截取

protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
Configuration configuration;
XMLConfigBuilder xmlConfigBuilder = null;

...

if (!isEmpty(this.mapperLocations)) {
for (Resource mapperLocation : this.mapperLocations) {
if (mapperLocation == null) {
continue;
}
try {
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
configuration, mapperLocation.toString(), configuration.getSqlFragments());
xmlMapperBuilder.parse();
} catch (Exception e) {
throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
} finally {
ErrorContext.instance().reset();
}
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Parsed mapper file: '" + mapperLocation + "'");
}
}
} else {
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Property 'mapperLocations' was not specified or no matching resources found");
}
}
return this.sqlSessionFactoryBuilder.build(configuration);
}

513行:for (Resource mapperLocation : this.mapperLocations) 循环解析Mapper内容519行:XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(...) 解析XMLMapperBuilder521行:xmlMapperBuilder.parse() 执行解析,具体如下;

>XMLMapperBuilder.java & 部分截取

public class XMLMapperBuilder extends BaseBuilder {
private final XPathParser parser;
private final MapperBuilderAssistant builderAssistant;
private final Map sqlFragments;
private final String resource;

private void bindMapperForNamespace() {
String namespace = builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class boundType = null;
try {
boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException e) {
//ignore, bound type is not required
}
if (boundType != null) {
if (!configuration.hasMapper(boundType)) {
// Spring may not know the real resource name so we set a flag
// to prevent loading again this resource from the mapper interface
// look at MapperAnnotationBuilder#loadXmlResource
configuration.addLoadedResource("namespace:" + namespace);
configuration.addMapper(boundType);
}
}
}
}
}

这里413行非常重要,configuration.addMapper(boundType);,真正到了添加Mapper到配置中心

>MapperRegistry.java & 部分截取

public class MapperRegistry {

public void addMapper(Class type) {
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
knownMappers.put(type, new MapperProxyFactory(type));
// It's important that the type is added before the parser is run
// otherwise the binding may automatically be attempted by the
// mapper parser. If the type is already known, it won't try.
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}

}

67行:创建代理工程knownMappers.put(type, new MapperProxyFactory(type));

截至到这,MapperScannerConfigurer、SqlSessionFactoryBean,两个类干的事情就相融合了;

第一个用于扫描Dao接口设置代理类注册到IOC中,用于后续生成Bean实体类,MapperFactoryBean,并可以通过mapperInterface从Configuration获取Mapper另一个用于生成SqlSession工厂初始化,解析Mapper里的XML配置进行动态代理MapperProxyFactory->MapperProxy注入到Configuration的Mapper最终在注解类的帮助下进行方法注入,等执行操作时候即可获得动态代理对象,从而执行相应的CRUD操作

@Resource
private ISchoolDao schoolDao;

schoolDao.querySchoolInfoById(1L);

六、综上总结

分析过程较长篇幅也很大,不一定一天就能看懂整个流程,但当耐下心来一点点研究,还是可以获得很多的收获的。以后在遇到这类的异常就可以迎刃而解了,同时也有助于面试、招聘!之所以分析Mybatis最开始是想在Dao上加自定义注解,发现切面拦截不到。想到这是被动态代理的类,之后层层往往下扒直到MapperProxy.invoke!当然,Mybatis提供了自定义插件开发。以上的源码分析只是对部分核心内容进行分析,如果希望了解全部可以参考资料;MyBatis 3源码深度解析,并调试代码。IDEA中还是很方便看源码的,包括可以查看类图、调用顺序等。mybatis、mybatis-spring中其实最重要的是将Mapper配置文件解析与接口类组装成代理类进行映射,以此来方便对数据库的CRUD操作。从源码分析后,可以获得更多的编程经验(套路)。Mybatis相关链接;

- https://github.com/mybatis/mybatis-3"

- https://mybatis.org/mybatis-3/zh/index.html"

七、推荐阅读

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