Java 注解实例
package com.annotation; import java.lang.annotation.Retention; import java.lang.annotation.Target; import static java.lang.annotation.ElementType.*; import static java.lang.annotation.RetentionPolicy.*; /** * * @author cong * 数据库连接注解 * */ @Target(value = {FIELD})//Target:此注解用到的地方有TYPE、FIELD、METHOD @Retention(value = RUNTIME)//注解的运行环境 public @interface CustomConnection { String url() default ""; String driverClass() default ""; String username() default ""; String password() default ""; }
注解处理
package com.annotation.processor; import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.sql.Connection; import java.sql.DriverManager; import com.annotation.CustomConnection; @SuppressWarnings("unchecked")//不显示警告提示 public class CustomAnnotationProcessor { public static <T> void processor(Class<T> clazz) { try { //T t = clazz.newInstance(); T t = null; Field[] fields = clazz.getDeclaredFields();//获取T对象声明变量 Constructor<?>[] constructors = clazz.getDeclaredConstructors();//获取T对象的构造函数,因为构造函数的访问级别问题 for(Constructor<?> constructor : constructors) { constructor.setAccessible(true);//无论T对象的构造函数是什么访问级别都可以访问其方法 t = (T)constructor.newInstance(new Object[]{}); } for(Field field : fields) { if(field.isAnnotationPresent(CustomConnection.class)) { CustomConnection customConnection = field.getAnnotation(CustomConnection.class); String url = customConnection.url(); String driverClass = customConnection.driverClass(); String username = customConnection.username(); String password = customConnection.password(); Class.forName(driverClass); Connection connection = DriverManager.getConnection(url, username, password); field.setAccessible(true); field.set(t, connection); continue; } } } catch (Exception e) { e.printStackTrace(); } } }
package com.util; import java.sql.Connection; import com.annotation.CustomConnection; /** * * @author Administrator * 调用注解连接数据库 * */ public class DBUtil { private DBUtil() {//构造方法 } @CustomConnection(url = "jdbc:oracle:thin:@//192.168.40.29:1521/PDBMYORACLE", driverClass = "oracle.jdbc.driver.OracleDriver", username = "victop", password = "victop") private static Connection connection; public static Connection getConnection() { return connection; } }
测试
package com.victop; import java.sql.Connection; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.sql.SQLException; import com.annotation.processor.CustomAnnotationProcessor; import com.util.DBUtil; public class Test { public static void main(String[] args) { try { CustomAnnotationProcessor.processor(DBUtil.class); Connection connection = DBUtil.getConnection(); PreparedStatement preparedStatement = connection.prepareStatement("select * from Users"); ResultSet resultSet = preparedStatement.executeQuery(); while(resultSet.next()) { System.out.println(resultSet.getString("username")); } } catch (SQLException e) { e.printStackTrace(); } } }
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