java - 变量声明后的这个静态 block 是什么？

Question

我以前从未见过这个 - 它叫什么？这是一个类级变量，位于文件的开头。

为了清楚起见，我指的是变量后的 static {}。

private static final UriMatcher URI_MATCHER;
    static {
        URI_MATCHER = new UriMatcher(UriMatcher.NO_MATCH);
        URI_MATCHER.addURI(AUTHORITY, SearchManager.SUGGEST_URI_PATH_QUERY, SEARCH);
        URI_MATCHER.addURI(AUTHORITY, SearchManager.SUGGEST_URI_PATH_QUERY + "/*", SEARCH);
        URI_MATCHER.addURI(AUTHORITY, "books", BOOKS);
        URI_MATCHER.addURI(AUTHORITY, "books/#", BOOK_ID);
    }

Answer 1

这是一个静态初始化 block 。它可以在类内的任何地方(但在方法外)声明，但按照惯例，它通常写在被初始化的静态变量之后。它在 Java 语言规范中指定，section §8.7 .

顾名思义，它通常用于在类加载时初始化类中静态属性的状态。来自Java tutorial :

A static initialization block is a normal block of code enclosed in braces, { }, and preceded by the static keyword (...) A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code.