java - 如何使静态初始化 block 执行？

Question

在寻找为什么我的静态初始化 block 无法执行的解释时，我发现类中的静态初始化程序永远不会执行 if it references a final .

因此，我尝试通过从 MYWS_PROPS 中删除“final”来解决此问题:

public class HibernateUtil {
  public static String MYWS_PROPS = "/myws.properties";

    private static final Logger LOG = Logger.getLogger(HibernateUtil.class.getName());
    private static final SessionFactory sessionFactory = buildSessionFactory();
    private static Properties sProps;

    static {
        try {
            sProps = new Properties();
            sProps.load(ServiceUtils.class.getResourceAsStream(MYWS_PROPS));
            LOG.info("Loaded (from " + MYWS_PROPS + ") connection url: " + sProps.getProperty("dburl"));
        } 
        catch (IOException e) {
            LOG.severe("Cannot find properties file " + MYWS_PROPS);
        }               
    }   



    private static SessionFactory buildSessionFactory() {
        try {
            Configuration config = new Configuration();
            config = config.configure("resources/hibernate.cfg.xml");
            config.setProperty("hibernate.connection.url", sProps.getProperty("dburl"));    // <== NullPointerException!        
            SessionFactory session = config.buildSessionFactory();
            return session;
        }
        catch (Throwable ex) {
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

}

但是那个静态 block 仍然不会执行!

保证静态 block 执行的正确方法是什么？

<小时/>

更新:下面的所有答案都提到了引用该类的要求，以便静态初始化能够运行。因此，我再次查看堆栈跟踪并验证该类确实被引用(否则，为什么它会在同一个类方法中抛出异常？)

Caused by: java.lang.ExceptionInInitializerError
        at com.corp.dept.proj.myws.HibernateUtil.buildSessionFactory(HibernateUtil.java:76)
        at com.corp.dept.proj.myws.HibernateUtil.<clinit>(HibernateUtil.java:38)
        at com.corp.dept.proj.myws.ServicePortImpl.<init>(ServicePortImpl.java:82)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
        at java.lang.reflect.Constructor.newInstance(Unknown Source)
        at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:126)
        ... 70 more
Caused by: java.lang.NullPointerException
        at com.corp.dept.proj.myws.HibernateUtil.buildSessionFactory(HibernateUtil.java:67)
        ... 77 more

但为了安全起见，我尝试了 Class.forName 建议:

Class.forName("com.corp.dept.proj.myws.HibernateUtil");
mProps.load(ServiceUtils.class.getResourceAsStream(HibernateUtil.MYWS_PROPS));

那个静态 block 仍然不会执行。

所以我尝试显式实例化它:

HibernateUtil justExecTheDarnStaticBlock = new HibernateUtil();
mProps.load(ServiceUtils.class.getResourceAsStream(HibernateUtil.MYWS_PROPS));

但是那个静态 block 仍然不会执行!

我在这里缺少什么？

Answer 1

处理静态初始化时，顺序很重要。

看下面的例子:

public class StaticTest {
    private static final String someString = initSomeString();
    private static final String someOtherString = initSomeOtherString();
    private static final String whoops;

    static {
        System.out.println("static initializer");
        whoops = "whoops";
    }

    public static void main(String[] args) {
        System.out.println(someOtherString);
    }

    private static String initSomeString() {
        System.out.println("initSomeString");
        return "Initialised";
    }

    private static String initSomeOtherString() {
        System.out.println("initSomeOtherString");
        return whoops + "y daisy";
    }
}

运行该类的结果如下:

initSomeString
initSomeOtherString
static initialiser
nully daisy

静态初始化按照声明的顺序发生，这意味着 initSomeOtherString() 在初始化的静态初始化 block 之前被调用哎呀哎呀。因此，此时 whoops 仍然是 null，因此生成的 String 是 nully daisy 而不是 百日菊。

如果我重新排列类，以便在静态初始化器之后初始化 someOtherString ，我会得到我想要的:

public class StaticTest {
    private static final String someString = initSomeString();
    private static final String whoops;

    static {
        System.out.println("static initialiser");
        whoops = "whoops";
    }

    private static final String someOtherString = initSomeOtherString();

    public static void main(String[] args) {
        System.out.println(someOtherString);
    }

    private static String initSomeString() {
        System.out.println("initSomeString");
        return "Initialised";
    }

    private static String initSomeOtherString() {
        System.out.println("initSomeOtherString");
        return whoops + "y daisy";
    }
}

打印

initSomeString
static initialiser
initSomeOtherString
whoopsy daisy

这是您遇到的行为并导致了您的问题。您通过调用 buildSessionFactory() 来初始化 sessionFactory，这依赖于初始化 sProps:

config.setProperty("hibernate.connection.url", sProps.getProperty("dburl")
                                               ^ This hasn't been initialised yet

解决方案？ 将 sessionFactory 的声明移至静态初始化程序 block 的下方。