　　用了好久的Java了，从来没有看过jdk的源码，趁着今天有点时间，拿出了jdk的源码看了下，今天先看了关于String的，毕竟开发中String类型使用最广泛。在我们下载安装jdk的时候，部分源码也已经同时存放在我们电脑里了，具体路径为jdk目录下的src.zip压缩包，解压即可。

java.lang.String

1 public final class String 2 implements java.io.Serializable, Comparable<String>, CharSequence

这是String类的声明，很明显它是由final类型声明的，所以它不能被继承，而它又实现了Serializable接口，代表它是可以被序列化的。

接着我们来看看类内部是怎么定义的

/** The value is used for character storage. */

    private final char value[];

使用final类型的字符数组存储字符串内容，String初始化后就不能被改变。

有一种写法，String s = “abc”； s = “bcd”；当然这并不是改变了字符串s的值，只是将s指向了一个新的字符串，所以千万不要以为字符串是可以变的。

/** Cache the hash code for the string */

    private int hash; // Default to 0

指定缓存字符串的hash code的值，默认为0

/** use serialVersionUID from JDK 1.0.2 for interoperability */

    private static final long serialVersionUID = -6849794470754667710L;

相当于java类的身份证。主要用于版本控制。
serialVersionUID作用是序列化时保持版本的兼容性，即在版本升级时反序列化仍保持对象的唯一性。
有两种生成方式：
       一个是默认的1L，比如：private static final long serialVersionUID = 1L;
       一个是根据类名、接口名、成员方法及属性等来生成一个64位的哈希字段，比如：
       private static final long serialVersionUID = xxxxL;

/**

     * Class String is special cased within the Serialization Stream Protocol.

     *

     * A String instance is written into an ObjectOutputStream according to

     * <a href="{@docRoot}/../platform/serialization/spec/output.html">

     * Object Serialization Specification, Section 6.2, "Stream Elements"</a>

     */

    private static final ObjectStreamField[] serialPersistentFields =

        new ObjectStreamField[0];

serialPersistentFields 用于指定哪些字段需要被默认序列化，如：

private static final ObjectStreamField[] serialPersistentFields =

{

    new ObjectStreamField("name", String.class),

    new ObjectStreamField("a", Integer.TYPE)

};

看了String类的构造方法，惊叹不已，源码中有足足15种构造方法，其中最常用的不外乎以下几种

public String(byte bytes[]) {

        this(bytes, 0, bytes.length);

    }

public String(StringBuffer buffer) {

        synchronized(buffer) {

            this.value = Arrays.copyOf(buffer.getValue(), buffer.length());

        }

    }

public String(StringBuilder builder) {

        this.value = Arrays.copyOf(builder.getValue(), builder.length());

    }

再往后看就是String类自身提供的一系列方法了:

// 返回字符串的长度

    public int length() {

        return value.length;

    }

    // 字符串是否为空

    public boolean isEmpty() {

        return value.length == 0;

    }

    // 字符串目标位置的字符

    public char charAt(int index) {

        if ((index < 0) || (index >= value.length)) {

            throw new StringIndexOutOfBoundsException(index);

        }

        return value[index];

    }

    // 返回指定索引处的字符

    public int codePointAt(int index) {

        if ((index < 0) || (index >= value.length)) {

            throw new StringIndexOutOfBoundsException(index);

        }

        return Character.codePointAtImpl(value, index, value.length);

    }

    // 返回指定索引之前的字符

    public int codePointBefore(int index) {

        int i = index - 1;

        if ((i < 0) || (i >= value.length)) {

            throw new StringIndexOutOfBoundsException(index);

        }

        return Character.codePointBeforeImpl(value, index, 0);

    }

    // 返回此 String 的指定文本范围中的 Unicode 代码点数

    public int codePointCount(int beginIndex, int endIndex) {

        if (beginIndex < 0 || endIndex > value.length || beginIndex > endIndex) {

            throw new IndexOutOfBoundsException();

        }

        return Character.codePointCountImpl(value, beginIndex, endIndex - beginIndex);

    }

//返回从0处开始的第i个Code Point的位置

public int offsetByCodePoints(int index, int codePointOffset) {

        if (index < 0 || index > value.length) {

            throw new IndexOutOfBoundsException();

        }

        return Character.offsetByCodePointsImpl(value, 0, value.length,

                index, codePointOffset);

    }

    //判断两个字符串是否相等

public boolean equals(Object anObject) {

        if (this == anObject) {

            return true;

        }

        if (anObject instanceof String) {

            String anotherString = (String)anObject;

            int n = value.length;

            if (n == anotherString.value.length) {

                char v1[] = value;

                char v2[] = anotherString.value;

                int i = 0;

                while (n-- != 0) {

                    if (v1[i] != v2[i])

                        return false;

                    i++;

                }

                return true;

            }

        }

        return false;

    }

    //不区分大小写的情况下比较两个字符串是否相等

public boolean equalsIgnoreCase(String anotherString) {

        return (this == anotherString) ? true

                : (anotherString != null)

                && (anotherString.value.length == value.length)

                && regionMatches(true, 0, anotherString, 0, value.length);

    }

//计算当前字符串比目标字符串长度大多少

public int compareTo(String anotherString) {

        int len1 = value.length;

        int len2 = anotherString.value.length;

        int lim = Math.min(len1, len2);

        char v1[] = value;

        char v2[] = anotherString.value;

        int k = 0;

        while (k < lim) {

            char c1 = v1[k];

            char c2 = v2[k];

            if (c1 != c2) {

                return c1 - c2;

            }

            k++;

        }

        return len1 - len2;

    }

//获取哈希值

public int hashCode() {

        int h = hash;

        if (h == 0 && value.length > 0) {

            char val[] = value;

            for (int i = 0; i < value.length; i++) {

                h = 31 * h + val[i];

            }

            hash = h;

        }

        return h;

    }

//返回某个指定的字符串值在字符串中首次出现的位置

public int indexOf(int ch) {

        return indexOf(ch, 0);

    }

public int indexOf(int ch, int fromIndex) {

        final int max = value.length;

        if (fromIndex < 0) {

            fromIndex = 0;

        } else if (fromIndex >= max) {

            // Note: fromIndex might be near -1>>>1.

            return -1;

        }

        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {

            // handle most cases here (ch is a BMP code point or a

            // negative value (invalid code point))

            final char[] value = this.value;

            for (int i = fromIndex; i < max; i++) {

                if (value[i] == ch) {

                    return i;

                }

            }

            return -1;

        } else {

            return indexOfSupplementary(ch, fromIndex);

        }

    }

//返回某个指定的字符串值在字符串中最后一次出现的位置

public int lastIndexOf(int ch) {

        return lastIndexOf(ch, value.length - 1);

    }

public int lastIndexOf(int ch, int fromIndex) {

        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {

            // handle most cases here (ch is a BMP code point or a

            // negative value (invalid code point))

            final char[] value = this.value;

            int i = Math.min(fromIndex, value.length - 1);

            for (; i >= 0; i--) {

                if (value[i] == ch) {

                    return i;

                }

            }

            return -1;

        } else {

            return lastIndexOfSupplementary(ch, fromIndex);

        }

    }

//从指定位置截取字符串直至最后一位

public String substring(int beginIndex) {

        if (beginIndex < 0) {

            throw new StringIndexOutOfBoundsException(beginIndex);

        }

        int subLen = value.length - beginIndex;

        if (subLen < 0) {

            throw new StringIndexOutOfBoundsException(subLen);

        }

        return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);

    }

//截取字符串从beginIndex至beginIndex

public String substring(int beginIndex, int endIndex) {

        if (beginIndex < 0) {

            throw new StringIndexOutOfBoundsException(beginIndex);

        }

        if (endIndex > value.length) {

            throw new StringIndexOutOfBoundsException(endIndex);

        }

        int subLen = endIndex - beginIndex;

        if (subLen < 0) {

            throw new StringIndexOutOfBoundsException(subLen);

        }

        return ((beginIndex == 0) && (endIndex == value.length)) ? this

                : new String(value, beginIndex, subLen);

    }

//字符串拼接

public String concat(String str) {

        int otherLen = str.length();

        if (otherLen == 0) {

            return this;

        }

        int len = value.length;

        char buf[] = Arrays.copyOf(value, len + otherLen);

        str.getChars(buf, len);

        return new String(buf, true);

 }

//字符串替换

public String replace(char oldChar, char newChar) {

        if (oldChar != newChar) {

            int len = value.length;

            int i = -1;

            char[] val = value; /* avoid getfield opcode */

            while (++i < len) {

                if (val[i] == oldChar) {

                    break;

                }

            }

            if (i < len) {

                char buf[] = new char[len];

                for (int j = 0; j < i; j++) {

                    buf[j] = val[j];

                }

                while (i < len) {

                    char c = val[i];

                    buf[i] = (c == oldChar) ? newChar : c;

                    i++;

                }

                return new String(buf, true);

            }

        }

        return this;

    }

//判断字符串是否包含在另一个字符串中

public boolean contains(CharSequence s) {

        return indexOf(s.toString()) > -1;

    }

//将目标字符串中匹配的字符全部替换成另一字符串

public String replaceAll(String regex, String replacement) {

        return Pattern.compile(regex).matcher(this).replaceAll(replacement);

    }

//按约定分隔符将目标字符串分割成多个字符串并返回字符串数组

public String[] split(String regex, int limit) {

        /* fastpath if the regex is a

         (1)one-char String and this character is not one of the

            RegEx's meta characters ".$|()[{^?*+\\", or

         (2)two-char String and the first char is the backslash and

            the second is not the ascii digit or ascii letter.

         */

        char ch = 0;

        if (((regex.value.length == 1 &&

             ".$|()[{^?*+\\".indexOf(ch = regex.charAt(0)) == -1) ||

             (regex.length() == 2 &&

              regex.charAt(0) == '\\' &&

              (((ch = regex.charAt(1))-'0')|('9'-ch)) < 0 &&

              ((ch-'a')|('z'-ch)) < 0 &&

              ((ch-'A')|('Z'-ch)) < 0)) &&

            (ch < Character.MIN_HIGH_SURROGATE ||

             ch > Character.MAX_LOW_SURROGATE))

        {

            int off = 0;

            int next = 0;

            boolean limited = limit > 0;

            ArrayList<String> list = new ArrayList<>();

            while ((next = indexOf(ch, off)) != -1) {

                if (!limited || list.size() < limit - 1) {

                    list.add(substring(off, next));

                    off = next + 1;

                } else {    // last one

                    //assert (list.size() == limit - 1);

                    list.add(substring(off, value.length));

                    off = value.length;

                    break;

                }

            }

            // If no match was found, return this

            if (off == 0)

                return new String[]{this};

            // Add remaining segment

            if (!limited || list.size() < limit)

                list.add(substring(off, value.length));

            // Construct result

            int resultSize = list.size();

            if (limit == 0) {

                while (resultSize > 0 && list.get(resultSize - 1).length() == 0) {

                    resultSize--;

                }

            }

            String[] result = new String[resultSize];

            return list.subList(0, resultSize).toArray(result);

        }

        return Pattern.compile(regex).split(this, limit);

    }

巴特西

学习JDK源码（一）：String

java.lang.String

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