Java语言中字符串类型和字节数组类型相互之间的转换经常发生，网上的分析及代码也比较多，本文将分析总结常规的byte[]和String间的转换以及十六进制String和byte[]间相互转换的原理及实现。
1. String转byte[]
首先我们来分析一下常规的String转byte[]的方法，代码如下：
public static byte[] strToByteArray(String str) { if (str == null) { return null; } byte[] byteArray = str.getBytes(); return byteArray;} 很简单，就是调用String类的getBytes()方法。看JDK源码可以发现该方法最终调用了String类如下的方法。
/** * 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; }} 上述代码其实就是根据给定的编码方式进行编码。如果调用的是不带参数的getBytes()方法，则使用默认的编码方式，如下代码所示：
/** * 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"); }} 关于默认的编码方式，Java API是这样说的：
The default charset is determined during virtual-machine startup and typically depends upon the locale and charset of the underlying operating system.
同样，由上述代码可以看出，默认编码方式是由System类的"file.encoding"属性决定的，经过测试，在简体中文Windows操作系统下，默认编码方式为"GBK"，在Android平台上，默认编码方式为"UTF-8"。
2. byte[]转String
接下来分析一下常规的byte[]转为String的方法，代码如下：
public static String byteArrayToStr(byte[] byteArray) { if (byteArray == null) { return null; } String str = new String(byteArray); return str;} 很简单，就是String的构造方法之一。那我们分析Java中String的源码，可以看出所有以byte[]为参数的构造方法最终都调用了如下代码所示的构造方法。需要注意的是Java中String类的数据是Unicode类型的，因此上述的getBytes()方法是把Unicode类型转化为指定编码方式的byte数组；而这里的Charset为读取该byte数组时所使用的编码方式。
/** * 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-0000007F int 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 b0 int 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 back continue outer; } // Push new bits in from the right side val <<= 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 only if ((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 pairs if (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-0xff v[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; } }} 具体的转换过程较为复杂，其实就是将byte数组的一个或多个元素按指定的Charset类型读取并转换为char类型（char本身就是以Unicode编码方式存储的），因为String类的核心是其内部维护的char数组。因此有兴趣的同学可以研究下各种编码方式的编码规则，然后才能看懂具体的转换过程。
3. byte[]转十六进制String
所谓十六进制String，就是字符串里面的字符都是十六进制形式，因为一个byte是八位，可以用两个十六进制位来表示，因此，byte数组中的每个元素可以转换为两个十六进制形式的char，所以最终的HexString的长度是byte数组长度的两倍。闲话少说上代码：
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);} 上述代码中，之所以要将byte数值和0xFF按位与，是因为我们为了方便后面的无符号移位操作（无符号右移运算符>>>只对32位和64位的值有意义），要将byte数据转换为int类型，而如果直接转换就会出现问题。因为java里面二进制是以补码形式存在的，如果直接转换，位扩展会产生问题，如值为-1的byte存储的二进制形式为其补码11111111，而转换为int后为11111111111111111111111111111111，直接使用该值结果就不对了。而0xFF默认是int类型，即0x000000FF，一个byte值跟0xFF相与会先将那个byte值转化成int类型运算，这样，相与的结果中高的24个比特就总会被清0，后面的运算才会正确。
4. 十六进制String转byte[]
没什么好说的了，就是byte[]转十六进制String的逆过程，放代码：
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;}