java - 如何使用 Derby SQLState 常量

Question

我的 Java 应用程序正在与 Apache Derby 10.8.1.2 数据库通信。我捕获了 SQL 异常并想针对特定情况进行测试:LANG_DUPLICATE_KEY_CONSTRAINT我找不到合适的 .jar 来导入据称包含此常量的 org.apache.derby.shared.common.reference。

甚至 Derby Developer's Guide 也建议像这样进行测试:

if ( SQLState.equals( "23505" ) ) 

问题是，为什么要测试静态字符串而不是常量？我希望是这样的:

if ( SQLState.equals( SQLState.LANG_DUPLICATE_KEY_CONSTRAINT ) ) 

Answer 1

你可能不喜欢这个答案......但它是:

SqlState 故意不包含在任何 jar 文件中，以尽量减少 Derby 的大小。引用包说明:

Package org.apache.derby.shared.common.reference Description

Useful constants that the Java compiler will in-line. A collection of reference interfaces that contain primitive or String constants. Such constants will be in-lined by the Java compiler hopefully leading to smaller footprint, than a reference to a field entails. Fields in interfaces are automatically public, static and final, hence constants.

These classes should not need to be shipped with any of the Derby jar files. If one is shipped with a Jar file then it most likely means the field representing the constant was a non-String Object (e.g. new Integer(1)) or the final field is a calculated value that could not be resolved to a constant by the Java compiler.