我编写了一个测试程序，它创建了一个Derby连接，并创建了必要的表，如果它们不存在的话。但是一旦我运行该程序，我就无法重新运行它，因为我带来的例外。我认为我应该为当前的单例类添加一个finilize方法并关闭那里的连接。这是解决方案吗？
但无论如何我认为程序崩溃的可能性以及以异常方式退出程序。在这种情况下，我如何强制安全地关闭与数据库的其他连接实例并建立自己的连接？
public class DBTest {
public static void main(String[] args) {
DBConnection.getInstance();
}
}
public class DBConnection {
private static DBConnection ourInstance = new DBConnection();
private Connection connection;
public static DBConnection getInstance() {
return ourInstance;
}
private DBConnection() {
String driver = "org.apache.derby.jdbc.EmbeddedDriver";
String dbName="C:/Users/Puk/IdeaProjects/DerbyTest/database/Ablauf";
String connectionURL = "jdbc:derby:" + dbName + ";create=true";
try {
Class.forName(driver);
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
try {
connection = DriverManager.getConnection(connectionURL);
} catch (SQLException e) {
e.printStackTrace();
}
createTables();
}
private void createTables(){
Statement s;
try {
s = connection.createStatement();
s.execute("CREATE TABLE category ( id SMALLINT NOT NULL GENERATED ALWAYS AS IDENTITY \n" +
"\t(START WITH 0, INCREMENT BY 1), title VARCHAR(100))");
} catch (SQLException e) {
e.printStackTrace();
}
}
}
java.sql.SQLException: Failed to start database 'C:/Users/Puk/IdeaProjects/DerbyTest/database/Ablauf' with class loader sun.misc.Launcher$AppClassLoader@1830e4a7, see the next exception for details.
at org.apache.derby.impl.jdbc.SQLExceptionFactory40.getSQLException(Unknown Source)
at org.apache.derby.impl.jdbc.Util.newEmbedSQLException(Unknown Source)
at org.apache.derby.impl.jdbc.Util.seeNextException(Unknown Source)
at org.apache.derby.impl.jdbc.EmbedConnection.bootDatabase(Unknown Source)
at org.apache.derby.impl.jdbc.EmbedConnection.<init>(Unknown Source)
at org.apache.derby.impl.jdbc.EmbedConnection40.<init>(Unknown Source)
at org.apache.derby.jdbc.Driver40.getNewEmbedConnection(Unknown Source)
at org.apache.derby.jdbc.InternalDriver.connect(Unknown Source)
at org.apache.derby.jdbc.Driver20.connect(Unknown Source)
at org.apache.derby.jdbc.AutoloadedDriver.connect(Unknown Source)
at java.sql.DriverManager.getConnection(DriverManager.java:579)
at java.sql.DriverManager.getConnection(DriverManager.java:243)
at model.DBConnection.<init>(DBConnection.java:31)
at model.DBConnection.<clinit>(DBConnection.java:12)
at test.DBTest.main(DBTest.java:11)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
Caused by: java.sql.SQLException: Failed to start database 'C:/Users/Puk/IdeaProjects/DerbyTest/database/Ablauf' with class loader sun.misc.Launcher$AppClassLoader@1830e4a7, see the next exception for details.
at org.apache.derby.impl.jdbc.SQLExceptionFactory.getSQLException(Unknown Source)
at org.apache.derby.impl.jdbc.SQLExceptionFactory40.wrapArgsForTransportAcrossDRDA(Unknown Source)
... 20 more
Caused by: java.sql.SQLException: Another instance of Derby may have already booted the database C:\Users\Puk\IdeaProjects\DerbyTest\database\Ablauf.
at org.apache.derby.impl.jdbc.SQLExceptionFactory.getSQLException(Unknown Source)
at org.apache.derby.impl.jdbc.SQLExceptionFactory40.wrapArgsForTransportAcrossDRDA(Unknown Source)
at org.apache.derby.impl.jdbc.SQLExceptionFactory40.getSQLException(Unknown Source)
at org.apache.derby.impl.jdbc.Util.generateCsSQLException(Unknown Source)
... 17 more
Caused by: ERROR XSDB6: Another instance of Derby may have already booted the database C:\Users\Puk\IdeaProjects\DerbyTest\database\Ablauf.
at org.apache.derby.iapi.error.StandardException.newException(Unknown Source)
at org.apache.derby.impl.store.raw.data.BaseDataFileFactory.privGetJBMSLockOnDB(Unknown Source)
at org.apache.derby.impl.store.raw.data.BaseDataFileFactory.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at org.apache.derby.impl.store.raw.data.BaseDataFileFactory.getJBMSLockOnDB(Unknown Source)
at org.apache.derby.impl.store.raw.data.BaseDataFileFactory.boot(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.boot(Unknown Source)
at org.apache.derby.impl.services.monitor.TopService.bootModule(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.startModule(Unknown Source)
at org.apache.derby.iapi.services.monitor.Monitor.bootServiceModule(Unknown Source)
at org.apache.derby.impl.store.raw.RawStore.boot(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.boot(Unknown Source)
at org.apache.derby.impl.services.monitor.TopService.bootModule(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.startModule(Unknown Source)
at org.apache.derby.iapi.services.monitor.Monitor.bootServiceModule(Unknown Source)
at org.apache.derby.impl.store.access.RAMAccessManager.boot(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.boot(Unknown Source)
at org.apache.derby.impl.services.monitor.TopService.bootModule(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.startModule(Unknown Source)
at org.apache.derby.iapi.services.monitor.Monitor.bootServiceModule(Unknown Source)
at org.apache.derby.impl.db.BasicDatabase.bootStore(Unknown Source)
at org.apache.derby.impl.db.BasicDatabase.boot(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.boot(Unknown Source)
at org.apache.derby.impl.services.monitor.TopService.bootModule(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.bootService(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.startProviderService(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.findProviderAndStartService(Unknown Source)
at org.apache.derby.impl.services.monitor.BaseMonitor.startPersistentService(Unknown Source)
at org.apache.derby.iapi.services.monitor.Monitor.startPersistentService(Unknown Source)
... 17 more
Exception in thread "main" java.lang.ExceptionInInitializerError
at test.DBTest.main(DBTest.java:11)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
Caused by: java.lang.NullPointerException
at model.DBConnection.createTables(DBConnection.java:41)
at model.DBConnection.<init>(DBConnection.java:35)
at model.DBConnection.<clinit>(DBConnection.java:12)
... 6 more
Process finished with exit code 1