Java9新特性系列（String改进）

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Java9前时代

在Java9之前，String的源码如下：

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package java.lang;
public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    /** The value is used for character storage. */
    private final char value[];
    ...
}

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, using two 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 characters require 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：

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package java.lang;
public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {

    /**
     * The value is used for character storage.
     *
     * @implNote This field is trusted by the VM, and is a subject to
     * constant folding if String instance is constant. Overwriting this
     * field after construction will cause problems.
     *
     * Additionally, it is marked with {@link Stable} to trust the contents
     * of the array. No other facility in JDK provides this functionality (yet).
     * {@link Stable} is safe here, because value is never null.
     */
    @Stable
    private final byte[] value;
    ...
}

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.

与String相关对比如AbstractStringBuilder、StringBuilder、StringBuffer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～