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假设我运行下面的代码

import java.util.ArrayList;
import java.util.List;

public class NoClassDefFoundErrorDueToStaticInitFailure {

    public static void main(String args[]) {
        System.out.println("Main start");
        new User("1");
    }
}

class User {
    private static String USER_ID = getUserId();

    public User(String id) {
        this.USER_ID = id;
    }

    private static String getUserId() {
        throw new RuntimeException("UserId Not found");
    }

}

它给

Main start
Exception in thread "main" java.lang.ExceptionInInitializerError
    at com.NoClassDefFoundError.NoClassDefFoundErrorDueToStaticInitFailure.main(NoClassDefFoundErrorDueToStaticInitFailure.java:10)
Caused by: java.lang.RuntimeException: UserId Not found
    at com.NoClassDefFoundError.User.getUserId(NoClassDefFoundErrorDueToStaticInitFailure.java:22)
    at com.NoClassDefFoundError.User.<clinit>(NoClassDefFoundErrorDueToStaticInitFailure.java:15)
    ... 1 more

但是，如果我从上面的 User 类中删除静态并使其如下所示

class User {
    private String USER_ID = getUserId();

    public User(String id) {
        this.USER_ID = id;
    }

    private String getUserId() {
        throw new RuntimeException("UserId Not found");
    }
}

堆栈跟踪现在是：

Main start
Exception in thread "main" java.lang.RuntimeException: UserId Not found
    at com.NoClassDefFoundError.User.getUserId(NoClassDefFoundErrorDueToStaticInitFailure.java:22)
    at com.NoClassDefFoundError.User.<init>(NoClassDefFoundErrorDueToStaticInitFailure.java:15)
    at com.NoClassDefFoundError.NoClassDefFoundErrorDueToStaticInitFailure.main(NoClassDefFoundErrorDueToStaticInitFailure.java:10)

由于删除了静态（类级别）关键字，无法理解为什么会发生这种变化

4

1 回答 1

2

在第一个示例中，当您到达线路时

new User("1");

类User被加载并初始化，执行

private static String USER_ID = getUserId();

由于投入ExceptionInInitializerError了. 由抛出。RuntimeExceptiongetUserId()ExceptionInInitializerErrorClassLoader

在第二个示例中，您只需有一个实例方法，它会抛出一个RuntimeException. 当您最终调用该方法时，它将冒泡到 JVM 的未捕获异常处理程序并终止程序。

考虑阅读 Java 语言规范一章的When Initialization Occurs。

于 2013-10-02T18:37:42.147 回答