mybatis独立使用及源码分析
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mybatis独立使用及源码分析
不使用mybatis使用jdbc
JdbcExample
public class JdbcExample {
public static void main(String[] args) throws Exception{
Class.forName("org.postgresql.Driver");
String url = "jdbc:postgresql://10.25.76.198:5432/artifact";
String username = "dev_root";
String password = "dev_root";
Connection connection = DriverManager.getConnection(url, username, password);
Statement stmt = connection.createStatement();
PreparedStatement preparedStatement = connection.prepareStatement("select * from clean_policy where id = ?");
preparedStatement.setLong(1,292L);
//prepare sql进行预处理
ResultSet resultSet = preparedStatement.executeQuery();
while (resultSet.next()) {
String name = resultSet.getString("name");
System.out.println("name:" + name);
}
//statement 不预处理直接替换
String sql = "select * from clean_policy where id = {0}";
sql = MessageFormat.format(sql, 292L);
ResultSet resultSet1 = stmt.executeQuery(sql);
while (resultSet1.next()) {
String name = resultSet1.getString("name");
System.out.println("name1:" + name);
}
}
}
我们直接使用jdbc查询语句有两种方式：
1. 预处理
2. 直接替换
两种方式的本质区别：预处理可以避免sql注入;直接替换无法避免预处理
SPI
Class.forName
//加载pg的驱动,这里加载类会触发类的静态代码块
Class.forName("org.postgresql.Driver");
org.postgresql.Driver
class Driver{
//org.postgresql.Driver类的静态代码块，再类加载的时候就执行
static {
try {
register();
} catch (SQLException var1) {
throw new ExceptionInInitializerError(var1);
}
}
public static void register() throws SQLException {
if (isRegistered()) {
throw new IllegalStateException("Driver is already registered. It can only be registered once.");
} else {
Driver registeredDriver = new Driver();
// 这里把驱动注册到驱动管理器 DriverManager的registeredDrivers集合中
DriverManager.registerDriver(registeredDriver);
Driver.registeredDriver = registeredDriver;
}
}
}
获取到数据库连接从已加载的驱动中获取
DriverManager.getConnection(url, username, password);
private static Connection getConnection(){
for(DriverInfo aDriver : registeredDrivers) {
Connection con = aDriver.driver.connect(url, info);
return con
}
}
SPI的机制
上面JdbcExample中main方法我们去掉Class.forName方法，发现还是可以正常获取数据连接，这说明数据库驱动是被
正常加载的，这种机制是如何实现的呢？ 这种就是我们接下来要了解的SPI机制(service provider interface)
public class DriverManager {
static {
loadInitialDrivers();
println("JDBC DriverManager initialized");
}
private static void loadInitialDrivers() {
while(driversIterator.hasNext()) {
driversIterator.next();
}
}
}
// 通过下面的源码我们可以看到driversIterator == lookupIterator
class ServiceLoader{
public static <S> ServiceLoader<S> load(Class<S> service) {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return ServiceLoader.load(service, cl);
}
public static <S> ServiceLoader<S> load(Class<S> service,
ClassLoader loader)
{
return new ServiceLoader<>(service, loader);
}
private ServiceLoader(Class<S> svc, ClassLoader cl) {
service = Objects.requireNonNull(svc, "Service interface cannot be null");
loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
reload();
}
public void reload() {
providers.clear();
lookupIterator = new LazyIterator(service, loader);
}
}
//driversIterator.hasNext()
private static final String PREFIX = "META-INF/services/";
private boolean hasNextService() {
if (configs == null) {
try {
// service.getName() == java.sql.Driver
String fullName = PREFIX + service.getName();
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
configs = loader.getResources(fullName);
} catch (IOException x) {
fail(service, "Error locating configuration files", x);
}
}
while ((pending == null) || !pending.hasNext()) {
if (!configs.hasMoreElements()) {
return false;
}
//读取到jar包org.postgresql的META-INF下的services的java.sql.Driver的内容org.postgresql.Driver
pending = parse(service, configs.nextElement());
}
nextName = pending.next();
return true;
}
// driversIterator.next();
private S nextService() {
// 这里nextName就是从hasNextService方法中读到的org.postgresql.Driver,
String cn = nextName;
//和加载org.postgresql.Driver类，执行静态代码块，把自己注册到DriverManager的registeredDrivers集合中
c = Class.forName(cn, false, loader);
}
总结：通过约定jar包具体目录存放接口的实现类，来动态可插拔加载类的方式
这里通过: 接口 + 配置文件 通过策略模式来实现spi机制,
- springboot中stater中就是通过这种方式来实现自动加载注入配置的。
- java中领域参数校验Validation也有采用这种机制
不依赖spring使用mybaits简单实例
创建Maven项目
MybatisUse
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.InputStream;
/**
* @author xuelongjiang109
* @description
**/
public class MybatisUse {
public static void main(String[] args) throws Exception{
String mybatisXml = "mybatis.xml";
InputStream inputStream = Resources.getResourceAsStream(mybatisXml);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
CleanPolicyMapper cleanPolicyMapper = sqlSession.getMapper(CleanPolicyMapper.class);
CleanPolicy cleanPolicy = cleanPolicyMapper.selectById(292L);
System.out.println("name:" + cleanPolicy.getName());
sqlSession.commit();
sqlSession.flushStatements();
sqlSession.close();
}
}
mybaits.xml
位于resources目录下
<?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 核心配置文件 -->
<configuration>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="org.postgresql.Driver"/>
<property name="url" value="jdbc:postgresql://10.25.76.198:5432/artifact"/>
<property name="username" value="dev_root"/>
<property name="password" value="dev_root"/>
</dataSource>
</environment>
</environments>
<!-- 每一个 Mapper.XML 都需要在 Mybatis 核心配置文件中注册！！-->
<mappers>
<mapper class="com.xuelongjiang.testanyone.mybatis.CleanPolicyMapper"/>
<!--<mapper class="com.song.dao.UserMapper"/>-->
<!--<package name="com.song.dao"/>-->
</mappers>
</configuration>
CleanPolicyMapper
import org.apache.ibatis.annotations.Select;
public interface CleanPolicyMapper {
@Select("select * from clean_policy where id = #{id} ")
CleanPolicy selectById(Long id);
}
CleanPolicy实体
/**
* @author xuelongjiang109
* @description
**/
public class CleanPolicy {
private Long id;
private String name;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
源码分析
下面源码会省略部分代码，我们只要焦距在主流程就行。
生成sql会话工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//sqlSessionFactory是sqlSession的工厂，sqlSession是对数据库一次会话的抽象
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
// parser.parse() 返回org.apache.ibatis.session.Configuration
return build(parser.parse());
}
//返回默认的会话工厂 DefaultSqlSessionFactory
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
parser.parse()
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
// 读取到xml的根节点configuration
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
// 解析xml中的节点及其内容
private void parseConfiguration(XNode root) {
environmentsElement(root.evalNode("environments"));
mapperElement(root.evalNode("mappers"));
// ....省略了解析其他节点如：properties,plugins,plugins
}
解析数据库配置
// 解析读取数据库配置 结果存放到configuration的environment属性
private void environmentsElement(XNode context) throws Exception {
if (context != null) {
if (environment == null) {
//得到指定数据源的id 对用我们配置的 development
environment = context.getStringAttribute("default");
}
// 读取environment节点的内容
for (XNode child : context.getChildren()) {
String id = child.getStringAttribute("id");
//如果当前环境的id等于指定的id则继续读取数据库相关配置
if (isSpecifiedEnvironment(id)) {
TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
DataSource dataSource = dsFactory.getDataSource();
Environment.Builder environmentBuilder = new Environment.Builder(id)
.transactionFactory(txFactory)
.dataSource(dataSource);
configuration.setEnvironment(environmentBuilder.build());
}
}
}
}
解析mapper
依次读取resource、url、class 注意这里如果mapper中的这个三个属性只能配置一个
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
//如果配置的是package则加载其下的所有类
if ("package".equals(child.getName())) {
String mapperPackage = child.getStringAttribute("name");
configuration.addMappers(mapperPackage);
} else {
// 如果是mapper 依次读取resource、url、class 注意这里如果mapper中的这个三个属性只能配置一个
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.");
}
}
}
}
}
// 继续解析mappers中的sql
mapperParser.parse();
// 解析xml中的select,iseret,update,delete语句
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
// 解析sql语句的各种配置以及Sql内容构造出MappedStatement来存储这些信息
statementParser.parseStatementNode();
//注意：MappedStatement中的id是namespace.id
MappedStatement
//MappedStatement接下来被放到configuration.mappedStatements中(这一个map StrictMap是mybatis继承HashMap重写了部分方法)
configuration.addMappedStatement(statement);
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