Java語言中字符串類型和字節數組類型相互之間的轉換經常發生，網上的分析及代碼也比較多，本文將分析總結常規的byte[]和String間的轉換以及十六進制String和byte[]間相互轉換的原理及實現。

1. String轉byte[]首先我們來分析一下常規的String轉byte[]的方法，代碼如下：public static byte[] strToByteArray(String str) {if (str == null) {return null;}byte[] byteArray = str.getBytes();return byteArray; }很簡單，就是調用String類的getBytes()方法。看JDK源碼可以發現該方法最終調用了String類如下的方法。/*** JDK source code*/ public byte[] getBytes(Charset charset) {String canonicalCharsetName = charset.name();if (canonicalCharsetName.equals("UTF-8")) {return Charsets.toUtf8Bytes(value, offset, count);} else if (canonicalCharsetName.equals("ISO-8859-1")) {return Charsets.toIsoLatin1Bytes(value, offset, count);} else if (canonicalCharsetName.equals("US-ASCII")) {return Charsets.toAsciiBytes(value, offset, count);} else if (canonicalCharsetName.equals("UTF-16BE")) {return Charsets.toBigEndianUtf16Bytes(value, offset, count);} else {CharBuffer chars = CharBuffer.wrap(this.value, this.offset, this.count);ByteBuffer buffer = charset.encode(chars.asReadOnlyBuffer());byte[] bytes = new byte[buffer.limit()];buffer.get(bytes);return bytes;} }上述代碼其實就是根據給定的編碼方式進行編碼。如果調用的是不帶參數的getBytes()方法，則使用默認的編碼方式，如下代碼所示：/*** JDK source code*/ private static Charset getDefaultCharset() {String encoding = System.getProperty("file.encoding", "UTF-8");try {return Charset.forName(encoding);} catch (UnsupportedCharsetException e) {return Charset.forName("UTF-8");} }關于默認的編碼方式，Java API是這樣說的：The default charset is determined during virtual-machine startup and typically depends upon the locale and charset of the underlying operating system.同樣，由上述代碼可以看出，默認編碼方式是由System類的"file.encoding"屬性決定的，經過測試，在簡體中文Windows操作系統下，默認編碼方式為"GBK"，在Android平臺上，默認編碼方式為"UTF-8"。 2. byte[]轉String接下來分析一下常規的byte[]轉為String的方法，代碼如下：public static String byteArrayToStr(byte[] byteArray) {if (byteArray == null) {return null;}String str = new String(byteArray);return str; }很簡單，就是String的構造方法之一。那我們分析Java中String的源碼，可以看出所有以byte[]為參數的構造方法最終都調用了如下代碼所示的構造方法。需要注意的是Java中String類的數據是Unicode類型的，因此上述的getBytes()方法是把Unicode類型轉化為指定編碼方式的byte數組；而這里的Charset為讀取該byte數組時所使用的編碼方式。/*** JDK source code*/ public String(byte[] data, int offset, int byteCount, Charset charset) {if ((offset | byteCount) < 0 || byteCount > data.length - offset) { throw failedBoundsCheck(data.length, offset, byteCount);}// We inline UTF-8, ISO-8859-1, and US-ASCII decoders for speed and because// 'count' and 'value' are final.String canonicalCharsetName = charset.name();if (canonicalCharsetName.equals("UTF-8")) {byte[] d = data;char[] v = new char[byteCount];int idx = offset;int last = offset + byteCount;int s = 0;outer:while (idx < last) {byte b0 = d[idx++];if ((b0 & 0x80) == 0) {// 0xxxxxxx// Range: U-00000000 - U-0000007Fint val = b0 & 0xff;v[s++] = (char) val;} else if (((b0 & 0xe0) == 0xc0) || ((b0 & 0xf0) == 0xe0) ||((b0 & 0xf8) == 0xf0) || ((b0 & 0xfc) == 0xf8) || ((b0 & 0xfe)== 0xfc)) {int utfCount = 1;if ((b0 & 0xf0) == 0xe0) utfCount = 2;else if ((b0 & 0xf8) == 0xf0) utfCount = 3;else if ((b0 & 0xfc) == 0xf8) utfCount = 4;else if ((b0 & 0xfe) == 0xfc) utfCount = 5;// 110xxxxx (10xxxxxx)+// Range: U-00000080 - U-000007FF (count == 1)// Range: U-00000800 - U-0000FFFF (count == 2)// Range: U-00010000 - U-001FFFFF (count == 3)// Range: U-00200000 - U-03FFFFFF (count == 4)// Range: U-04000000 - U-7FFFFFFF (count == 5)if (idx + utfCount > last) {v[s++] = REPLACEMENT_CHAR;continue;}// Extract usable bits from b0int val = b0 & (0x1f >> (utfCount - 1));for (int i = 0; i < utfCount; ++i) {byte b = d[idx++];if ((b & 0xc0) != 0x80) {v[s++] = REPLACEMENT_CHAR;idx--; // Put the input char backcontinue outer;}// Push new bits in from the right sideval <<= 6;val |= b & 0x3f;}// Note: Java allows overlong char// specifications To disallow, check that val// is greater than or equal to the minimum// value for each count://// count min value// ----- ----------// 1 0x80// 2 0x800// 3 0x10000// 4 0x200000// 5 0x4000000// Allow surrogate values (0xD800 - 0xDFFF) to// be specified using 3-byte UTF values onlyif ((utfCount != 2) && (val >= 0xD800) && (val <= 0xDFFF)) {v[s++] = REPLACEMENT_CHAR;continue;}// Reject chars greater than the Unicode maximum of U+10FFFF.if (val > 0x10FFFF) {v[s++] = REPLACEMENT_CHAR;continue;}// Encode chars from U+10000 up as surrogate pairsif (val < 0x10000) {v[s++] = (char) val;} else {int x = val & 0xffff;int u = (val >> 16) & 0x1f;int w = (u - 1) & 0xffff;int hi = 0xd800 | (w << 6) | (x >> 10);int lo = 0xdc00 | (x & 0x3ff);v[s++] = (char) hi;v[s++] = (char) lo;}} else {// Illegal values 0x8*, 0x9*, 0xa*, 0xb*, 0xfd-0xffv[s++] = REPLACEMENT_CHAR;}}if (s == byteCount) {// We guessed right, so we can use our temporary array as-is.this.offset = 0;this.value = v;this.count = s;} else {// Our temporary array was too big, so reallocate and copy.this.offset = 0;this.value = new char[s];this.count = s;System.arraycopy(v, 0, value, 0, s);}} else if (canonicalCharsetName.equals("ISO-8859-1")) {this.offset = 0;this.value = new char[byteCount];this.count = byteCount;Charsets.isoLatin1BytesToChars(data, offset, byteCount, value);} else if (canonicalCharsetName.equals("US-ASCII")) {this.offset = 0;this.value = new char[byteCount];this.count = byteCount;Charsets.asciiBytesToChars(data, offset, byteCount, value);} else {CharBuffer cb = charset.decode(ByteBuffer.wrap(data, offset, byteCount));this.offset = 0;this.count = cb.length();if (count > 0) {// We could use cb.array() directly, but that would mean we'd have to trust// the CharsetDecoder doesn't hang on to the CharBuffer and mutate it later,// which would break String's immutability guarantee. It would also tend to// mean that we'd be wasting memory because CharsetDecoder doesn't trim the// array. So we copy.this.value = new char[count];System.arraycopy(cb.array(), 0, value, 0, count);} else {this.value = EmptyArray.CHAR;}} }具體的轉換過程較為復雜，其實就是將byte數組的一個或多個元素按指定的Charset類型讀取并轉換為char類型（char本身就是以Unicode編碼方式存儲的），因為String類的核心是其內部維護的char數組。因此有興趣的同學可以研究下各種編碼方式的編碼規則，然后才能看懂具體的轉換過程。 3. byte[]轉十六進制String所謂十六進制String，就是字符串里面的字符都是十六進制形式，因為一個byte是八位，可以用兩個十六進制位來表示，因此，byte數組中的每個元素可以轉換為兩個十六進制形式的char，所以最終的HexString的長度是byte數組長度的兩倍。閑話少說上代碼：public static String byteArrayToHexStr(byte[] byteArray) {if (byteArray == null){return null;}char[] hexArray = "0123456789ABCDEF".toCharArray();char[] hexChars = new char[byteArray.length * 2];for (int j = 0; j < byteArray.length; j++) {int v = byteArray[j] & 0xFF;hexChars[j * 2] = hexArray[v >>> 4];hexChars[j * 2 + 1] = hexArray[v & 0x0F];}return new String(hexChars); }上述代碼中，之所以要將byte數值和0xFF按位與，是因為我們為了方便后面的無符號移位操作（無符號右移運算符>>>只對32位和64位的值有意義），要將byte數據轉換為int類型，而如果直接轉換就會出現問題。因為java里面二進制是以補碼形式存在的，如果直接轉換，位擴展會產生問題，如值為-1的byte存儲的二進制形式為其補碼11111111，而轉換為int后為11111111111111111111111111111111，直接使用該值結果就不對了。而0xFF默認是int類型，即0x000000FF，一個byte值跟0xFF相與會先將那個byte值轉化成int類型運算，這樣，相與的結果中高的24個比特就總會被清0，后面的運算才會正確。 4. 十六進制String轉byte[]沒什么好說的了，就是byte[]轉十六進制String的逆過程，放代碼：public static byte[] hexStrToByteArray(String str) {if (str == null) {return null;}if (str.length() == 0) {return new byte[0];}byte[] byteArray = new byte[str.length() / 2];for (int i = 0; i < byteArray.length; i++){String subStr = str.substring(2 * i, 2 * i + 2);byteArray[i] = ((byte)Integer.parseInt(subStr, 16));}return byteArray; }文中所有代碼可以在個人github主頁查看和下載。

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