Kotlin 数据库连接错误

Question

Exception in thread "main" kotlin.KotlinNullPointerException at Kotlin.main(DB.kt:4)

can anyone explain me what is this and what should i do to handle this exception?

object Kotlin {
    @JvmStatic fun main(args: Array<String>) {
        val conn: Connection = null!!
        val url = "jdbc:mysql://localhost:3306/"
        val dbName = "db-01"
        val driver = "com.mysql.jdbc.Driver"
        try {
            Class.forName(driver).newInstance()
            conn = DriverManager.getConnection("jdbc:mysql://localhost/student")
            println("Connected to the database")
            conn.close()
            println("Disconnected from database")
        } catch (e: Exception) {
            e.printStackTrace()
        }
    }
}

Answer 1

val conn: Connection = null!!

The !! 'casts' a nullable type to a non-null type, if the value is not null . If it is null , it throws a NullPointerException .

Thus, executing null!! results in a NullPointerException .
Instead, use a lateinit var , or make the Connection nullable.

Answer 2

It is perfectly fine not to initialize local val , if you're going to initialize it later:

fun main(args: Array<String>) {
    val conn: Connection
    // ... some code ...
    conn = DriverManager.getConnection("jdbc:mysql://localhost/student")
}

And if you do not need the connection outside of try block, you can declare it and initialize it at the same line:

try {
    //...
    val conn = DriverManager.getConnection("jdbc:mysql://localhost/student")
    //...
    conn.close()
} catch (e: Exception) {
    e.printStackTrace()
}