doc: 阅读r2dbc文档

spring/spring data r2dbc/r2dbc.md

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+# R2DBC
+## Getting Started
+### 引入pom
+在spring boot项目中，如果要适配spring data r2dbc，可以引入如下pom依赖：
+```xml
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-data-r2dbc</artifactId>
+        </dependency>
+
+        <dependency>
+            <groupId>io.asyncer</groupId>
+            <artifactId>r2dbc-mysql</artifactId>
+            <version>1.4.1</version>
+        </dependency>
+```
+### create pojo & create table
+之后，可以创建如下实体：
+```java
+public class Person {
+
+	private final String id;
+	private final String name;
+	private final int age;
+
+	public Person(String id, String name, int age) {
+		this.id = id;
+		this.name = name;
+		this.age = age;
+	}
+
+	public String getId() {
+		return id;
+	}
+
+	public String getName() {
+		return name;
+	}
+
+	public int getAge() {
+		return age;
+	}
+
+	@Override
+	public String toString() {
+		return "Person [id=" + id + ", name=" + name + ", age=" + age + "]";
+	}
+}
+```
+创建table：
+```sql
+CREATE TABLE person(
+    id VARCHAR(255) PRIMARY KEY,
+    name VARCHAR(255),
+    age INT
+);
+```
+### 声明ConnectionFactory
+```java
+@Configuration
+@Slf4j
+public class ConnectionFactoryConf extends AbstractR2dbcConfiguration {
+    @Bean
+    public ConnectionFactory connectionFactory() {
+//        ConnectionFactories.get("jdbc:mysql://localhost:3306/innodb_demo?useSSL=false&rewriteBatchedStatements=true")
+//                .
+        ConnectionFactoryOptions options = ConnectionFactoryOptions.builder()
+                .option(ConnectionFactoryOptions.HOST, "localhost")
+                .option(ConnectionFactoryOptions.PORT, 3306)
+                .option(ConnectionFactoryOptions.DRIVER, "mysql")
+                .option(ConnectionFactoryOptions.USER, "rw_all_db")
+                .option(ConnectionFactoryOptions.PASSWORD, "ab19971025")
+                .option(ConnectionFactoryOptions.DATABASE, "asahi_schedule")
+                .build();
+        ConnectionPoolConfiguration poolConf = ConnectionPoolConfiguration.builder()
+                .connectionFactory(ConnectionFactories.get(options))
+                .maxSize(10)
+                .validationQuery("SELECT 1")
+                .build();
+        return new ConnectionPool(poolConf);
+    }
+}
+```
+在上述示例中，Conf类继承了AbstractR2dbcConfiguration类，相比于直接注入`connectionFactory`对象，通过AbstractR2dbcConfiguration注册能够提供ExceptionTranslator，将r2dbc中的异常翻译为DataAccessException。
+
+### 通过R2dbcEntityTemplate操作数据库
+通过entityTemplate操作数据库的示例如下：
+```java
+R2dbcEntityTemplate template = new R2dbcEntityTemplate(connectionFactory);
+
+    template.getDatabaseClient().sql("CREATE TABLE person" +
+        "(id VARCHAR(255) PRIMARY KEY," +
+        "name VARCHAR(255)," +
+        "age INT)")
+      .fetch()
+      .rowsUpdated()
+      .as(StepVerifier::create)
+      .expectNextCount(1)
+      .verifyComplete();
+
+    template.insert(Person.class)
+      .using(new Person("joe", "Joe", 34))
+      .as(StepVerifier::create)
+      .expectNextCount(1)
+      .verifyComplete();
+
+    template.select(Person.class)
+      .first()
+      .doOnNext(it -> log.info(it))
+      .as(StepVerifier::create)
+      .expectNextCount(1)
+      .verifyComplete();
+  }
+```
+在上述示例中，pojo到table的映射会自动生效，并不需要额外的元数据。
+### Dialects
+spring data R2DBC会使用Dialect来对`特定数据库/driver的行为`进行封装。