Question

这可能看起来像一个重复的问题，但事实并非如此。我试着在48小时内找到答案而没有结果。

首先，关闭PreparedStatment＆amp; SQLite JDBC Connection中需要ResultSet吗？因为我无法这样做

try {
        Class.forName(database.getJDBC_DRIVER());
        cnn = DriverManager.getConnection(database.getDB_URL());
        p = cnn.prepareStatement(query);
        rs = p.executeQuery();
        p.close();
        cnn.close();
    } catch (SQLException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } finally {
        try {
            System.out.println(p.isClosed());
            if (cnn != null) cnn.close();
            System.out.println(p.isClosed());
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

如您所见，我在try块中关闭了PreparedStatement。但是，当我检查try块外部的状态时，结果总是为false。（这意味着它没有关闭）。

此外，如果我在finally块中有下面的代码，它会抛出一个错误，说明连接已关闭。对于该做什么，我感到非常困惑。我应该保持原样。我的代码不会有一些泄漏吗？

} finally {
    try {
        if (p != null) p.close(); *// It errors out here...*
        if (cnn != null) cnn.close();
    } catch (SQLException e) {
        e.printStackTrace();
    }
}

下面是堆栈跟踪 -

java.sql.SQLException: [SQLITE_ERROR] SQL error or missing database (Connection is closed)
    at org.sqlite.core.DB.newSQLException(DB.java:890)
    at org.sqlite.core.CoreStatement.internalClose(CoreStatement.java:109)
    at org.sqlite.jdbc3.JDBC3Statement.close(JDBC3Statement.java:35)
    at tg.cat.DropDown.getData(DropDown.java:28)
    at tg.loginscreen.LoginScreenLayout.<init>(LoginScreenLayout.java:22)
    at tg.cat.CatMain.getScene(CatMain.java:27)
    at tg.cat.CatMain.start(CatMain.java:18)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$159(Unknown Source)
    at com.sun.javafx.application.LauncherImpl$$Lambda$53/855499929.run(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$172(Unknown Source)
    at com.sun.javafx.application.PlatformImpl$$Lambda$45/186276003.run(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$null$170(Unknown Source)
    at com.sun.javafx.application.PlatformImpl$$Lambda$48/1635925971.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$171(Unknown Source)
    at com.sun.javafx.application.PlatformImpl$$Lambda$47/237061348.run(Unknown Source)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$145(Unknown Source)
    at com.sun.glass.ui.win.WinApplication$$Lambda$36/2117255219.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Answer 1

无论是否抛出异常，都会执行finally块。因此，您无需使用它来再次关闭连接，而是将其用作关闭连接的唯一位置。其次，请注意，关闭连接不会将变量重置为null：

try {
    Class.forName(database.getJDBC_DRIVER());
    cnn = DriverManager.getConnection(database.getDB_URL());
    p = cnn.prepareStatement(query);
    rs = p.executeQuery();
} catch (SQLException e) {
    e.printStackTrace();
} catch (ClassNotFoundException e) {
    e.printStackTrace();
} finally {
    try {
        if (rs != null) { 
            rs.close();
        }
        if (p != null) { 
            p.close();
        }
        if (cnn != null) { 
            cnn.close();
        }
    } catch (SQLException e) {
        System.err.println("Can't close an object, not much I can do");
    }
}

Answer 2

Mureinik是对的，但更好的是使用try-with-resources功能，这样你甚至不必担心首先关闭任何东西（手动）。

try (Connection cnn = DriverManager.getConnection(database.getDB_URL());
     PreparedStatement p = cnn.prepareStatement(query)) {
  // prepare your statement
  try (ResultSet rs = p.executeQuery()) {
    // process result set
  }
} catch (SQLException e) {
    throw new RuntimeException("Unexpected SQLException", e);
}

SQLite JDBC Connections的准确语法是什么？

2 个答案: