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JDK 9学习笔记 - (2)能屈能伸的String

背景

String作为JDK最核心的数据类型之一，非常有必要专门学习一下，重点关注这4个文件

存储

无论是何种语言的何种实现，String本质上都是字节序列，所有可能的字符加起来就构成了字符集，给字符集中每个字符一个序号就是字符编码，使用最广泛的就是Unicode了，它几乎支持地球上所有常见文字，Unicode有三种最主要的实现，UTF-8，UTF-16还有UTF-32，在web领域，UTF-8已经处于绝对垄断地位。Java 9的String，引入了类似Python str的压缩功能。原理很简单，如果String只包含Latin1字符，1字节存一个字符够用了，如果String含有中文，那么就换一种编码方式存储，用一个变量表示当前的字符集就行了。先看三个类。

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;

    /**
     * The identifier of the encoding used to encode the bytes in
     * {@code value}. The supported values in this implementation are
     *
     * LATIN1
     * UTF16
     *
     * @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.
     */
    private final byte coder;

    /** Cache the hash code for the string */
    private int hash; // Default to 0
}

final class StringLatin1 {
}

final class StringUTF16 {
}

字符序列存储在字节数组value中，然后用一个字节的coder表示编码，这是String的基本构成。然后StringLatin1提供一组静态方法，用来处理只含有Latin1字

编辑于 2017-10-31 00:18

Java 虚拟机（JVM）

Java

背景

存储

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