java单例使用示例

• Z时代
• 2024-01-10
• 分类：综合

/**
 * 立即加载/饿汉模式
 */
public class StarvingModelSingleton {
    private static StarvingModelSingleton instance = new StarvingModelSingleton();
    private StarvingModelSingleton() {
    }
    public static StarvingModelSingleton getInstance() {
        return instance;
    }
    public void doSomeThing() {
        System.out.println(">>> StarvingModelSingleton do some things");
    }
}

2、延迟加载/懒汉模式

/**
 * 延迟加载/懒汉模式
 */
public class SlackerModelSingleton {
    private static SlackerModelSingleton instance;
    private SlackerModelSingleton() {
    }
    public static SlackerModelSingleton getInstance() {
        if (instance == null) {
            synchronized (SlackerModelSingleton.class) {
                if (instance == null) {
                    instance = new SlackerModelSingleton();
                }
            }
        }
        return instance;
    }
    public void doSomeThing() {
        System.out.println(">>> SlackerModelSingleton do some things");
    }
}

3、用Enum实现单例示例

import java.sql.Connection;
import java.sql.DriverManager;
/**
 * 用Enum实现示例
 */
public class EnumModelSingleton {
    public static Connection getInstance() {
        return EnumSingleton.SingletonFactory.getConnection();
    }
    public enum EnumSingleton {
        SingletonFactory;
        private Connection connection;
        //枚举
        private EnumSingleton() {
            try {
                Class.forName("com.mysql.jdbc.Driver");
                connection = DriverManager.getConnection(
                        "jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=UTF-8&serverTimezone=UTC", "root", "123456");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        public Connection getConnection() {
            return connection;
        }
    }
}

使用示例

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
@RunWith(SpringRunner.class)
@SpringBootTest
public class SingletonTest {
    @Test
    public void test1() throws SQLException {
        StarvingModelSingleton.getInstance().doSomeThing();
        SlackerModelSingleton.getInstance().doSomeThing();
        Connection connection = EnumModelSingleton.getInstance();
        PreparedStatement preparedStatement = connection.prepareStatement("select * from t_user");
        ResultSet resultSet = preparedStatement.executeQuery();
        while (resultSet.next()){
            String userName = resultSet.getString("name");
            System.out.println(">>> " + userName);
        }
        resultSet.close();
        connection.close();
    }
}

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