java9新特性-11-String存储结构变更

1. 官方Feature

JEP254: Compact Strings

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2. 产生背景

Motivation

The current implementation of the String classstores characters in a char array,using two bytes (sixteen bits) for each character. Data gathered from manydifferent applications indicates that strings are a major component of heapusage and, moreover, that most String objectscontain only Latin-1 characters. Such characters require only one byte ofstorage, hence half of the space in the internal char arraysof such String objects is going unused.

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3. 使用说明

Description

We propose to change the internal representation of the Stringclass 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.

�结论：String 再也不用 char[] 来存储啦，改成了 byte[] 加上编码标记，节约了一些空间。

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4. 拓展：StringBuffer 与StringBuilder

那StringBuffer 和 StringBuilder 是否仍无动于衷呢？

�String-related classes such as AbstractStringBuilder, StringBuilder, and StringBuffer will

be updated to use the same representation, as will the HotSpot VM's intrinsic string operations.