通過列舉實現單例模式
阿新 • • 發佈:2019-01-31
實現單例的核心在於private私有化類中的構造方法,在列舉中的構造方法必須是私有的,當一個列舉類中只有一個列舉時只會呼叫一次列舉的構造方法,這就為列舉來實現單例奠定了基礎。下面以資料來源中獲得Connection連線來舉例:
在開發中經常是通過資料來源來獲得資料庫的Connection,資料來源的實現方式有多種,最典型的有兩種,一種是C3P0,另外一種是DBCP(以後有機會再針對兩種資料來源進行討論),這裡以C3P0資料來源為例。由於資料來源的建立與銷燬都是十分消耗效能的,並且系統中有一個數據源一般就能滿足開發的要求,因此要將資料來源設計為單例。
driverClass = com.mysql.jdbc.Driver jdbcUrl = jdbc:mysql://localhost:3306/liaokailin user = root password = mysqladmin maxPoolSize = 20 minPoolSize = 5
建立一個MyDataBaseSource的列舉:
package org.lkl.imitate_enum; import java.sql.Connection; import java.sql.SQLException; import java.util.ResourceBundle; import com.mchange.v2.c3p0.ComboPooledDataSource; public enum MyDataBaseSource { DATASOURCE; private ComboPooledDataSource cpds = null; private MyDataBaseSource() { try { /*--------獲取properties檔案內容------------*/ // 方法一: /* * InputStream is = * MyDBSource.class.getClassLoader().getResourceAsStream("jdbc.properties"); * Properties p = new Properties(); p.load(is); * System.out.println(p.getProperty("driverClass") ); */ // 方法二:(不需要properties的字尾) /* * ResourceBundle rb = PropertyResourceBundle.getBundle("jdbc") ; * System.out.println(rb.getString("driverClass")); */ // 方法三:(不需要properties的字尾) ResourceBundle rs = ResourceBundle.getBundle("jdbc"); cpds = new ComboPooledDataSource(); cpds = new ComboPooledDataSource(); cpds.setDriverClass(rs.getString("driverClass")); cpds.setJdbcUrl(rs.getString("jdbcUrl")); cpds.setUser(rs.getString("user")); cpds.setPassword(rs.getString("password")); cpds.setMaxPoolSize(Integer.parseInt(rs.getString("maxPoolSize"))); cpds.setMinPoolSize(Integer.parseInt(rs.getString("minPoolSize"))); System.out.println("-----呼叫了構造方法------"); ; } catch (Exception e) { e.printStackTrace(); } } public Connection getConnection() { try { return cpds.getConnection(); } catch (SQLException e) { return null; } } }
測試程式碼:
package org.lkl.imitate_enum;
public class Test {
public static void main(String[] args) {
MyDataBaseSource.DATASOURCE.getConnection() ;
MyDataBaseSource.DATASOURCE.getConnection() ;
MyDataBaseSource.DATASOURCE.getConnection() ;
}
}
結果如下:
-----呼叫了構造方法------ 2013-7很顯然獲得了三個Connection連線,但是隻呼叫了一次列舉的構造方法,從而通過列舉實現了單例的設計-17 17:10:57 com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource getPoolManager 資訊: Initializing c3p0 pool... com.mchange.v2.c3p0.ComboPooledDataSource [ acquireIncrement -> 3, acquireRetryAttempts -> 30, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, dataSourceName -> 1hge16d8v1tgb0wppydrzz|2c1e6b, debugUnreturnedConnectionStackTraces -> false, description -> null, driverClass -> com.mysql.jdbc.Driver, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, identityToken -> 1hge16d8v1tgb0wppydrzz|2c1e6b, idleConnectionTestPeriod -> 0, initialPoolSize -> 3, jdbcUrl -> jdbc:mysql://localhost:3306/kaoqin, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 0, maxIdleTimeExcessConnections -> 0, maxPoolSize -> 20, maxStatements -> 0, maxStatementsPerConnection -> 0, minPoolSize -> 5, numHelperThreads -> 3, preferredTestQuery -> null, properties -> {user=******, password=******}, propertyCycle -> 0, statementCacheNumDeferredCloseThreads -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, usesTraditionalReflectiveProxies -> false ]