Java9新特性系列

Java9前时代

在Java9之前，String的源码如下：

可以看到，String的内部是一个char数组，每个字符占2个字节（16位）。

Java9

官方Feature给出了如下说明：

产生背景：

The current implementation of the String class stores characters in a char array, usingtwo bytes (sixteen bits)for each character. Data gathered from many different applications indicates that strings are a major component of heap usage and, moreover, that most String objects contain only Latin-1 characters. Such charactersrequire only one byte of storage, hence half of the space in the internal char arrays of such String objects is going unused.

大量数据表明，String对象占用了主要的堆使用，而且，大部分的字符串对象只包含Latin-1字符，这样的字符只需要一个字节的存储空间，因此此类字符串对象的内部char数组中的一半空间将被闲置。所以，Java9中对String对存储结构进行了改进：

描述：

We propose to change the internal representation of the String class from a UTF-16 char array to a byte array plus an encoding-flag field.The new String class will store characters encoded either as ISO-8859-1/Latin-1 (one byte per character), or as UTF-16 (two bytes per character), based upon the contents of the string.The encoding flag will indicate which encoding is used.

String改成了byte数组，再加上编码标记，就节约了不少空间。

Java9中String源码如下：

String-related classes such as AbstractStringBuilder, StringBuilder, and StringBufferwill be updatedto use the same representation, as will the HotSpot VM’s intrinsic string operations.

与String相关对比如、、也将有相同的实现，不知道在3月份即将发布的Java10中实现呢，期待～

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