在java语言中用来表示字符串，所有的字符串（比如“abc”）都String的实例对象。String是常量，一旦创建不可以被修改，可以使用StringBuffer创建可变字符串。String类提供了字符串比较，查找，拷贝，大小写转换等操作。大小写转换基于标准的Unicode.  字符串拼接”+”：根据不同版本的jdk会有不同实现，如StringBuilder、StringBuffer、StringConcatFactory（invokeDynamic）

1. java.io.Serializable2. Comparable<String>3. CharSequence 提供对字符数组多种只读形式的统一访问方法规范

/** * jdk9开始使用byte[]存储字符串，1.8及之前使用char[]保存 */@Stableprivate final byte[] value;
/** *  coder用来表示此字符串使用的编码，coder=0使用LATIN1，coder=1使用UTF16 * *  LATIN1 是8比特的字符集，定义了256个字符。前128个字符与ASCII完全一致，即为ASCII的超集 *  UTF16  是可变长度编码。可以是一个或二个16比特。 *  根据不同的编码由不同的工具类实现String的内部编码，Latin1对应StringLatin1，UTF16对应StringUTF16 * */private final byte coder;
/** Cache the hash code for the string */private int hash; // Default to 0
/** use serialVersionUID from JDK 1.0.2 for interoperability */private static final long serialVersionUID = -6849794470754667710L;
/** * 如果关闭压缩，字符串的bytes使用UTF16编码 * * 如下为jit优化方面，为什么不直接初始化COMPACT_STRINGS的值： * * The instance field value is generally opaque to optimizing JIT * compilers. Therefore, in performance-sensitive place, an explicit * check of the static boolean {@code COMPACT_STRINGS} is done first * before checking the {@code coder} field since the static boolean * {@code COMPACT_STRINGS} would be constant folded away by an * optimizing JIT compiler. The idioms for these cases are as follows. * * For code such as: * *    if (coder == LATIN1) { ... } * * can be written more optimally as * *    if (coder() == LATIN1) { ... } * * or: * *    if (COMPACT_STRINGS && coder == LATIN1) { ... } * * An optimizing JIT compiler can fold the above conditional as: * *    COMPACT_STRINGS == true  => if (coder == LATIN1) { ... } *    COMPACT_STRINGS == false => if (false)           { ... } * * @implNote * The actual value for this field is injected by JVM. The static * initialization block is used to set the value here to communicate * that this static final field is not statically foldable, and to * avoid any possible circular dependency during vm initialization. * 事实上，COMPACT_STRINGS的值是由JVM填充的 */static final boolean COMPACT_STRINGS;
static {    COMPACT_STRINGS = true;}
/** * Class String is special cased within the Serialization Stream Protocol. * * A String instance is written into an ObjectOutputStream according to * <a href="{@docRoot}/../specs/serialization/protocol.html#stream-elements"> * Object Serialization Specification, Section 6.2, "Stream Elements"</a> */private static final ObjectStreamField[] serialPersistentFields =    new ObjectStreamField[0];    @Native static final byte LATIN1 = 0;@Native static final byte UTF16  = 1;

        1. getBytes()相关    /**    *  getBytes() 将当前字符串转换为当前文件系统默认编码格式的字节数组    *  getBytes(charset) 将当前字符串转换为指定编码格式的字节数组    */    public byte[] getBytes(String charsetName)            throws UnsupportedEncodingException {        if (charsetName == null) throw new NullPointerException();        return StringCoding.encode(charsetName, coder(), value);    }    public byte[] getBytes(Charset charset) {        if (charset == null) throw new NullPointerException();        return StringCoding.encode(charset, coder(), value);     }    public byte[] getBytes() {        return StringCoding.encode(coder(), value);    }    2. length()    /**    * 返回当前字符串长度，如果是LATIN1字符串长度等于LATIN1格式字节数组长度，否则需要取value.length>>1，长度减半    */    public int length() {        return value.length >> coder();    }    3. native intern()    /**     * 当调用intern方法时，如果常量池中已经存在equal当前String的对象，那么返回String常量池中的字符串。     * 否则，当前String对象会被添加到String常量池并且返回常量池中的String对象引用     * 如果a.intern() == b.intern(),那么a.equal(b) == true     */    public native String intern();