### [Spring Conditional Annotations](https://www.baeldung.com/spring-conditional-annotations)
```java
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;
import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.SQLException;
public class SecondaryDataSourceCondition implements Condition {
@Override
public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
try {
DataSource dataSource = context.getBeanFactory().getBean("secondaryDataSource", DataSource.class);
Connection connection = dataSource.getConnection();
connection.close();
return true;
} catch (SQLException ex) {
return false;
}
}
}
```
```java
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;
import org.springframework.jdbc.datasource.DriverManagerDataSource;
import javax.sql.DataSource;
@Configuration
public class DataSourceConfig {
@Bean(name = "primaryDataSource")
public DataSource primaryDataSource() {
DriverManagerDataSource dataSource = new DriverManagerDataSource();
dataSource.setUrl("jdbc:mysql://localhost:3306/primary_db");
dataSource.setUsername("primary_user");
dataSource.setPassword("primary_pass");
return dataSource;
}
@Bean(name = "secondaryDataSource")
@Conditional(SecondaryDataSourceCondition.class)
public DataSource secondaryDataSource() {
DriverManagerDataSource dataSource = new DriverManagerDataSource();
dataSource.setUrl("jdbc:mysql://localhost:3306/secondary_db");
dataSource.setUsername("secondary_user");
dataSource.setPassword("secondary_pass");
return dataSource;
}
}
```
### [The Spring @ConditionalOnProperty Annotation](https://www.baeldung.com/spring-conditionalonproperty)
```java
@Configuration
public class MyConfig {
@Bean
@ConditionalOnProperty(name = "my.property.enabled", havingValue = "true")
public MyBean myBean() {
return new MyBean();
}
}
```
在此示例中，當 my.property.enabled 屬性被設置為 true 時，myBean Bean 將被創建。如果屬性設置為其他值，則不會創建該 Bean。