无法在 gradle 任务中加载类“org.postgresql.Driver”

如何解决无法在 gradle 任务中加载类“org.postgresql.Driver”

我正在使用 gradle 任务来执行 sql：

我有一个 buildscript 依赖项：

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("postgresql:postgresql:9.0-801.jdbc4")
    }
}

这是我的任务（我使用的是 gradle kotlin DSL）：

tasks.register("deleteDB") {
    val driverName = "org.postgresql.Driver"
    Class.forName(driverName)
    val sql : sql = sql.newInstance(
            "jdbc:postgresql://localhost:5555/postgres","postgres",driverName
    )
    sql.execute("DROP DATABASE mydb;")
}

当我运行此任务时，出现此错误：

Unable to load class 'org.postgresql.Driver'.
Possible causes for this unexpected error include:
Gradle's dependency cache may be corrupt (this sometimes occurs after a network connection timeout.)
Re-download dependencies and sync project (requires network)

The state of a Gradle build process (daemon) may be corrupt. Stopping all Gradle daemons may solve this problem.
Stop Gradle build processes (requires restart)

Your project may be using a third-party plugin which is not compatible with the other plugins in the project or the version of Gradle requested by the project.

In the case of corrupt Gradle processes,you can also try closing the IDE and then killing all Java processes.

我做错了什么？