Java-try-catch内的变量范围-大多数答案与Java官方教程之间的对比

Question

我正在阅读以下official guide，但发现一个问题，将这两个代码片段一起使用将导致错误（stmt对象无作用域：]

处理ResultSet对象

try {
    stmt = con.createStatement();
    ResultSet rs = stmt.executeQuery(query);
    while (rs.next()) {
        String coffeeName = rs.getString("COF_NAME");
        int supplierID = rs.getInt("SUP_ID");
        float price = rs.getFloat("PRICE");
        int sales = rs.getInt("SALES");
        int total = rs.getInt("TOTAL");
        System.out.println(coffeeName + "\t" + supplierID +
                           "\t" + price + "\t" + sales +
                           "\t" + total);
    }
}

关闭连接

} finally {
    if (stmt != null) { stmt.close(); }
}

[如果我尝试在finally块中输入stmt.close()，我会得到一个错误，因为他的作用域中没有stmt变量，这是因为（据我所知）stmt对象的实际作用域在尝试块。

我的问题很简单，这两个代码块可以一起工作吗？

我发现的答案是否定的，仅将stmt对象的实例移动到try块之外会导致一个有效的代码段。

有人能给我他的想法吗？我只想了解我是否仍不清楚该论点的某些方面。

非常感谢任何人都会尝试帮助我。

Answer 1

大概，您已经在stmt内部而不是外部声明了try对象。代码应如下所示：

Statement stmt;
try {
    stmt = con.createStatement();
    ResultSet rs = stmt.executeQuery(query);
    while (rs.next()) {
        String coffeeName = rs.getString("COF_NAME");
        int supplierID = rs.getInt("SUP_ID");
        float price = rs.getFloat("PRICE");
        int sales = rs.getInt("SALES");
        int total = rs.getInt("TOTAL");
        System.out.println(coffeeName + "\t" + supplierID +
                           "\t" + price + "\t" + sales +
                           "\t" + total);
    }
} finally {
    if (stmt != null) { stmt.close(); }
}

但是，您可以使用Statement实现AutoClosable并实际上通过使用try with resources省略了finally块这一事实（Java 6 iirc中添加了Java功能）。看起来像：
try (Statement stmt = con.createStatement()) {
    ResultSet rs = stmt.executeQuery(query);
    while (rs.next()) {
        String coffeeName = rs.getString("COF_NAME");
        int supplierID = rs.getInt("SUP_ID");
        float price = rs.getFloat("PRICE");
        int sales = rs.getInt("SALES");
        int total = rs.getInt("TOTAL");
        System.out.println(coffeeName + "\t" + supplierID +
                           "\t" + price + "\t" + sales +
                           "\t" + total);
    }
} catch (SQLException e) {
    JDBCTutorialUtilities.printSQLException(e);
}

您可以在此处阅读有关AutoClosable和try with resources的更多信息：
https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

Answer 2

为了使之明确，您应该将stmt分配为null，以便它存在并被初始化：

Statement stmt = null;
try {
    stmt = con.createStatement();
    ResultSet rs = stmt.executeQuery(query);
    while (rs.next()) {
        String coffeeName = rs.getString("COF_NAME");
        int supplierID = rs.getInt("SUP_ID");
        float price = rs.getFloat("PRICE");
        int sales = rs.getInt("SALES");
        int total = rs.getInt("TOTAL");
        System.out.println(coffeeName + "\t" + supplierID +
                           "\t" + price + "\t" + sales +
                           "\t" + total);
    }
} finally {
    if (stmt != null) { stmt.close(); }
}