源码分析 | Mybatis接口没有实现类为什么可以执行增删改查

发布于: 6 小时前
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

博客:https://bugstack.cn - 汇总系列原创专题文章

沉淀、分享、成长,让自己和他人都能有所收获!😄

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

  1. JDK1.8

  2. IDEA 2019.3.1

  3. mybatis 3.4.6 {不同版本源码略有差异和bug修复}

  4. 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

<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.4.6</version>
</dependency>

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

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://127.0.0.1:3306/itstack?useUnicode=true"/>
<property name="username" value="root"/>
<property name="password" value="123456"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="mapper/User_Mapper.xml"/>
</mappers>
</configuration>

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:

  • 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);
}
...
}

  1. 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)

  2. 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;
}
}

  1. 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

  2. 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);
}
}

  1. 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

  2. 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;
}

  1. 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();
}
}

  1. 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;

  1. 属性解析propertiesElement

  2. 加载settings节点settingsAsProperties

  3. 载自定义VFS loadCustomVfs

  4. 解析类型别名typeAliasesElement

  5. 加载插件pluginElement

  6. 加载对象工厂objectFactoryElement

  7. 创建对象包装器工厂objectWrapperFactoryElement

  8. 加载反射工厂reflectorFactoryElement

  9. 元素设置settingsElement

  10. 加载环境配置environmentsElement

  11. 数据库厂商标识加载databaseIdProviderElement

  12. 加载类型处理器typeHandlerElement

  13. (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;

<!ELEMENT configuration (properties?, settings?, typeAliases?, typeHandlers?, objectFactory?, objectWrapperFactory?, reflectorFactory?, plugins?, environments?, databaseIdProvider?, mappers?)>
<!ELEMENT databaseIdProvider (property*)>
<!ATTLIST databaseIdProvider
type CDATA #REQUIRED
>
<!ELEMENT properties (property*)>
<!ATTLIST properties
resource CDATA #IMPLIED
url CDATA #IMPLIED
>
<!ELEMENT property EMPTY>
<!ATTLIST property
name CDATA #REQUIRED
value CDATA #REQUIRED
>
<!ELEMENT settings (setting+)>
<!ELEMENT setting EMPTY>
<!ATTLIST setting
name CDATA #REQUIRED
value CDATA #REQUIRED
>
<!ELEMENT typeAliases (typeAlias*,package*)>
<!ELEMENT typeAlias EMPTY>
<!ATTLIST typeAlias
type CDATA #REQUIRED
alias CDATA #IMPLIED
>
<!ELEMENT typeHandlers (typeHandler*,package*)>
<!ELEMENT typeHandler EMPTY>
<!ATTLIST typeHandler
javaType CDATA #IMPLIED
jdbcType CDATA #IMPLIED
handler CDATA #REQUIRED
>
<!ELEMENT objectFactory (property*)>
<!ATTLIST objectFactory
type CDATA #REQUIRED
>
<!ELEMENT objectWrapperFactory EMPTY>
<!ATTLIST objectWrapperFactory
type CDATA #REQUIRED
>
<!ELEMENT reflectorFactory EMPTY>
<!ATTLIST reflectorFactory
type CDATA #REQUIRED
>
<!ELEMENT plugins (plugin+)>
<!ELEMENT plugin (property*)>
<!ATTLIST plugin
interceptor CDATA #REQUIRED
>
<!ELEMENT environments (environment+)>
<!ATTLIST environments
default CDATA #REQUIRED
>
<!ELEMENT environment (transactionManager,dataSource)>
<!ATTLIST environment
id CDATA #REQUIRED
>
<!ELEMENT transactionManager (property*)>
<!ATTLIST transactionManager
type CDATA #REQUIRED
>
<!ELEMENT dataSource (property*)>
<!ATTLIST dataSource
type CDATA #REQUIRED
>
<!ELEMENT mappers (mapper*,package*)>
<!ELEMENT mapper EMPTY>
<!ATTLIST mapper
resource CDATA #IMPLIED
url CDATA #IMPLIED
class CDATA #IMPLIED
>
<!ELEMENT package EMPTY>
<!ATTLIST package
name CDATA #REQUIRED
>

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

  1. properties?,

  2. settings?,

  3. typeAliases?,

  4. typeHandlers?,

  5. objectFactory?,

  6. objectWrapperFactory?,

  7. reflectorFactory?,

  8. plugins?,

  9. environments?,

  10. databaseIdProvider?,

  11. 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 <? extends Log> 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 <? extends VFS> 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<String> lazyLoadTriggerMethods = new HashSet<String>(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<V> extends HashMap<String, V> {
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<String, ? extends V> 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:

<mappers>
<package name="org.itstack.demo"/>
</mappers>

  • 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;

<mappers>
<mapper resource="mapper/User_Mapper.xml"/>
<mapper class=""/>
<mapper url=""/>
</mappers>

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 {
...
}
}
}
}

Mapper加载到生成代理对象的流程中,主要的核心类包括;

  1. XMLConfigBuilder

  2. Configuration

  3. MapperRegistry

  4. MapperAnnotationBuilder

  5. MapperProxyFactory

>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<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
// 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<Class<? extends Class<?>>> 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 <T> void addMapper(Class<T> 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<T>(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<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

>MapperProxyFactory.java

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

<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis-spring</artifactId>
<version>1.3.2</version>
</dependency>

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

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<!-- 1.数据库连接池: DriverManagerDataSource 也可以使用DBCP2-->
<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="driverClassName" value="${db.jdbc.driverClassName}"/>
<property name="url" value="${db.jdbc.url}"/>
<property name="username" value="${db.jdbc.username}"/>
<property name="password" value="${db.jdbc.password}"/>
</bean>
<!-- 2.配置SqlSessionFactory对象 -->
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<!-- 注入数据库连接池 -->
<property name="dataSource" ref="dataSource"/>
<!-- 配置MyBaties全局配置文件:mybatis-config.xml -->
<property name="configLocation" value="classpath:mybatis-config.xml"/>
<!-- 扫描entity包 使用别名 -->
<property name="typeAliasesPackage" value="org.itstack.demo.po"/>
<!-- 扫描sql配置文件:mapper需要的xml文件 -->
<property name="mapperLocations" value="classpath:mapper/*.xml"/>
</bean>
<!-- 3.配置扫描Dao接口包,动态实现Dao接口,注入到spring容器中 -->
<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
<!-- 注入sqlSessionFactory -->
<property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
<!-- 给出需要扫描Dao接口包,多个逗号隔开 -->
<property name="basePackage" value="org.itstack.demo.dao"/>
</bean>
</beans>

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.SqlSessionFactoryBean

  • org.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:

  • BeanDefinitionRegistryPostProcessor

  • InitializingBean

  • ApplicationContextAware

  • BeanNameAware

整体类图如下;

执行流程如下;

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<BeanDefinitionHolder> doScan(String... basePackages)