前言
- 表示不变的通用唯一标识符（UUID）的类。 UUID表示一个128位的值。
- 这些全局标识符存在不同的变体。此类的方法用于操纵Leach-Salz变体，尽管构造函数允许创建UUID的任何变体（如下所述）。
- 变体2（Leach-Salz）UUID的布局如下：
- 最重要的long由以下无符号字段组成：
- 0xFFFFFFFF00000000 time_low
- 0x00000000FFFF0000 time_mid
- 0x000000000000F000 version
- 0x0000000000000FFF time_hi
- 最不重要的long由以下无符号字段组成：
- 0xC000000000000000 variant
- 0x3FFF000000000000 clock_seq
- 0x0000FFFFFFFFFFFF node
- 变量字段包含一个值，该值标识{@code UUID}的布局。
- 上述位布局仅对 variant value 为2（表示Leach-Salz 变种）的{@code UUID}有效。
- 版本字段包含一个描述此{@code UUID}类型的值。
- UUID有四种不同的基本类型：基于时间的，DCE安全性，基于名称的和随机生成的UUID。
- 这些类型的版本值分别为1、2、3和4。
- 有关包括用于创建{@code UUID}的算法的更多信息，请参见http://www.ietf.org/rfc/rfc4122.txt RFC 4122：
- 通用唯一标识符（UUID）URN命名空间，第4.2节“用于创建基于时间的UUID的算法”。
源码
package java.util;
public final class UUID implements java.io.Serializable, Comparable<UUID> {
/**
* Explicit serialVersionUID for interoperability.
*/
private static final long serialVersionUID = -4856846361193249489L;
/*
* The most significant 64 bits of this UUID.
*
* @serial
*/
private final long mostSigBits;
/*
* The least significant 64 bits of this UUID.
*
* @serial
*/
private final long leastSigBits;
/*
* The random number generator used by this class to create random
* based UUIDs. In a holder class to defer initialization until needed.
*/
private static class Holder {
static final SecureRandom numberGenerator = new SecureRandom();
}
// Constructors and Factories
/*
* Private constructor which uses a byte array to construct the new UUID.
*/
private UUID(byte[] data) {
long msb = 0;
long lsb = 0;
assert data.length == 16 : "data must be 16 bytes in length";
for (int i=0; i<8; i++)
msb = (msb << 8) | (data[i] & 0xff);
for (int i=8; i<16; i++)
lsb = (lsb << 8) | (data[i] & 0xff);
this.mostSigBits = msb;
this.leastSigBits = lsb;
}
/**
* Constructs a new {@code UUID} using the specified data. {@code
* mostSigBits} is used for the most significant 64 bits of the {@code
* UUID} and {@code leastSigBits} becomes the least significant 64 bits of
* the {@code UUID}.
*
* 使用指定的数据构造一个新的{@code UUID}。
* {@code mostSigBits}用于{@code UUID}的最高有效64位，并且{@code minimumSigBits}成为{@code UUID}的最低有效64位。
*
* @param mostSigBits
* The most significant bits of the {@code UUID}
*
* @param leastSigBits
* The least significant bits of the {@code UUID}
*/
public UUID(long mostSigBits, long leastSigBits) {
this.mostSigBits = mostSigBits;
this.leastSigBits = leastSigBits;
}
/**
* Static factory to retrieve a type 4 (pseudo randomly generated) UUID.
*
* The {@code UUID} is generated using a cryptographically strong pseudo
* random number generator.
*
* 静态工厂检索类型4（伪随机生成）的UUID。 {@code UUID}是使用密码学强的伪随机数生成器生成的。
*
* @return A randomly generated {@code UUID}
*/
public static UUID randomUUID() {
SecureRandom ng = Holder.numberGenerator;
byte[] randomBytes = new byte[16];
ng.nextBytes(randomBytes);
randomBytes[6] &= 0x0f; /* clear version */
randomBytes[6] |= 0x40; /* set to version 4 */
randomBytes[8] &= 0x3f; /* clear variant */
randomBytes[8] |= 0x80; /* set to IETF variant */
return new UUID(randomBytes);
}
/**
* Static factory to retrieve a type 3 (name based) {@code UUID} based on
* the specified byte array.
*
* 静态工厂根据指定的字节数组检索类型3（基于名称）{@ code UUID}。
* @param name
* A byte array to be used to construct a {@code UUID}
*
* @return A {@code UUID} generated from the specified array
*/
public static UUID nameUUIDFromBytes(byte[] name) {
MessageDigest md;
try {
md = MessageDigest.getInstance("MD5");
} catch (NoSuchAlgorithmException nsae) {
throw new InternalError("MD5 not supported", nsae);
}
byte[] md5Bytes = md.digest(name);
md5Bytes[6] &= 0x0f; /* clear version */
md5Bytes[6] |= 0x30; /* set to version 3 */
md5Bytes[8] &= 0x3f; /* clear variant */
md5Bytes[8] |= 0x80; /* set to IETF variant */
return new UUID(md5Bytes);
}
/**
* Creates a {@code UUID} from the string standard representation as
* described in the {@link
*
* 根据{@link
*
* @param name
* A string that specifies a {@code UUID}
*
* @return A {@code UUID} with the specified value
*
* @throws IllegalArgumentException
* If name does not conform to the string representation as
* described in {@link
*
*/
public static UUID fromString(String name) {
String[] components = name.split("-");
if (components.length != 5)
throw new IllegalArgumentException("Invalid UUID string: "+name);
for (int i=0; i<5; i++)
components[i] = "0x"+components[i];
long mostSigBits = Long.decode(components[0]).longValue();
mostSigBits <<= 16;
mostSigBits |= Long.decode(components[1]).longValue();
mostSigBits <<= 16;
mostSigBits |= Long.decode(components[2]).longValue();
long leastSigBits = Long.decode(components[3]).longValue();
leastSigBits <<= 48;
leastSigBits |= Long.decode(components[4]).longValue();
return new UUID(mostSigBits, leastSigBits);
}
// Field Accessor Methods
/**
* Returns the least significant 64 bits of this UUID's 128 bit value.
* 返回此UUID的128位值的最低有效64位。
* @return The least significant 64 bits of this UUID's 128 bit value
*/
public long getLeastSignificantBits() {
return leastSigBits;
}
/**
* Returns the most significant 64 bits of this UUID's 128 bit value.
* 返回此UUID的128位值的最高有效64位。 。
* @return The most significant 64 bits of this UUID's 128 bit value
*/
public long getMostSignificantBits() {
return mostSigBits;
}
/**
* The version number associated with this {@code UUID}. The version
* number describes how this {@code UUID} was generated.
*
* 与此{@code UUID}关联的版本号。版本号描述了如何生成此{@code UUID}。
* 版本号有如下意义：
* 1 Time-based UUID
* 2 DCE security UUID
* 3 Name-based UUID
* 4 Randomly generated UUID
*
* The version number has the following meaning:
* <ul>
* <li>1 Time-based UUID
* <li>2 DCE security UUID
* <li>3 Name-based UUID
* <li>4 Randomly generated UUID
* </ul>
*
* @return The version number of this {@code UUID}
*/
public int version() {
// Version is bits masked by 0x000000000000F000 in MS long
return (int)((mostSigBits >> 12) & 0x0f);
}
/**
* The variant number associated with this {@code UUID}. The variant
* number describes the layout of the {@code UUID}.
*
* 与此{@code UUID}关联的变体编号。变量号描述了{@code UUID}的布局
*
* 变体编号具有以下含义：
* 0保留用于NCS向后兼容
* 2 <a href="http://www.ietf.org/rfc/rfc4122.txt"> IETFRFC 4122（Leach-Salz），由此类使用
* 6保留，Microsoft Corporation向后兼容
* 7保留以供将来定义
*
* The variant number has the following meaning:
* <ul>
* <li>0 Reserved for NCS backward compatibility
* <li>2 <a href="http://www.ietf.org/rfc/rfc4122.txt">IETF RFC 4122</a>
* (Leach-Salz), used by this class
* <li>6 Reserved, Microsoft Corporation backward compatibility
* <li>7 Reserved for future definition
* </ul>
*
* @return The variant number of this {@code UUID}
*/
public int variant() {
// This field is composed of a varying number of bits.
// 0 - - Reserved for NCS backward compatibility
// 1 0 - The IETF aka Leach-Salz variant (used by this class)
// 1 1 0 Reserved, Microsoft backward compatibility
// 1 1 1 Reserved for future definition.
return (int) ((leastSigBits >>> (64 - (leastSigBits >>> 62)))
& (leastSigBits >> 63));
}
/**
* The timestamp value associated with this UUID.
*
* <p> The 60 bit timestamp value is constructed from the time_low,
* time_mid, and time_hi fields of this {@code UUID}. The resulting
* timestamp is measured in 100-nanosecond units since midnight,
* October 15, 1582 UTC.
*
* <p> The timestamp value is only meaningful in a time-based UUID, which
* has version type 1. If this {@code UUID} is not a time-based UUID then
* this method throws UnsupportedOperationException.
*
* 与此UUID关联的时间戳记值。
* 60位时间戳值由该{@code UUID}的time_low，time_mid和time_hi字段构成。
* 自1582年10月15日午夜以来，所产生的时间戳以100纳秒为单位进行测量。
* 时间戳记值仅在版本类型为1的基于时间的UUID中才有意义。
* 如果此{@code UUID}不是基于时间的UUID，则此方法将引发UnsupportedOperationException。
*
* @throws UnsupportedOperationException
* If this UUID is not a version 1 UUID
* @return The timestamp of this {@code UUID}.
*/
public long timestamp() {
if (version() != 1) {
throw new UnsupportedOperationException("Not a time-based UUID");
}
return (mostSigBits & 0x0FFFL) << 48
| ((mostSigBits >> 16) & 0x0FFFFL) << 32
| mostSigBits >>> 32;
}
/**
* The clock sequence value associated with this UUID.
*
* <p> The 14 bit clock sequence value is constructed from the clock
* sequence field of this UUID. The clock sequence field is used to
* guarantee temporal uniqueness in a time-based UUID.
*
* <p> The {@code clockSequence} value is only meaningful in a time-based
* UUID, which has version type 1. If this UUID is not a time-based UUID
* then this method throws UnsupportedOperationException.
*
* 与此UUID关联的时钟序列值。
* 14位时钟序列值是根据此UUID的时钟序列字段构造的。
* 时钟序列字段用于确保基于时间的UUID中的时间唯一性。
* {@code clockSequence}值仅在版本类型为1的基于时间的UUID中才有意义。
* 如果此UUID不是基于时间的UUID，则此方法将引发UnsupportedOperationException。
*
* @return The clock sequence of this {@code UUID}
*
* @throws UnsupportedOperationException
* If this UUID is not a version 1 UUID
*/
public int clockSequence() {
if (version() != 1) {
throw new UnsupportedOperationException("Not a time-based UUID");
}
return (int)((leastSigBits & 0x3FFF000000000000L) >>> 48);
}
/**
* The node value associated with this UUID.
*
* <p> The 48 bit node value is constructed from the node field of this
* UUID. This field is intended to hold the IEEE 802 address of the machine
* that generated this UUID to guarantee spatial uniqueness.
*
* <p> The node value is only meaningful in a time-based UUID, which has
* version type 1. If this UUID is not a time-based UUID then this method
* throws UnsupportedOperationException.
*
* 与此UUID关联的节点值。
* 48位节点值是从此UUID的node字段构造的。
* 该字段用于保存生成此UUID的机器的IEEE 802地址，以确保空间唯一性。
* 节点值仅在版本类型为1的基于时间的UUID中才有意义。
* 如果此UUID不是基于时间的UUID，则此方法将引发UnsupportedOperationException。
*
* @return The node value of this {@code UUID}
*
* @throws UnsupportedOperationException
* If this UUID is not a version 1 UUID
*/
public long node() {
if (version() != 1) {
throw new UnsupportedOperationException("Not a time-based UUID");
}
return leastSigBits & 0x0000FFFFFFFFFFFFL;
}
// Object Inherited Methods
/**
* Returns a {@code String} object representing this {@code UUID}.
* 返回表示此{@code UUID}的{@code String}对象。
* <p> The UUID string representation is as described by this BNF:
* <blockquote><pre>
* {@code
* UUID = <time_low> "-" <time_mid> "-"
* <time_high_and_version> "-"
* <variant_and_sequence> "-"
* <node>
* time_low = 4*<hexOctet>
* time_mid = 2*<hexOctet>
* time_high_and_version = 2*<hexOctet>
* variant_and_sequence = 2*<hexOctet>
* node = 6*<hexOctet>
* hexOctet = <hexDigit><hexDigit>
* hexDigit =
* "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
* | "a" | "b" | "c" | "d" | "e" | "f"
* | "A" | "B" | "C" | "D" | "E" | "F"
* }</pre></blockquote>
*
* @return A string representation of this {@code UUID}
*/
public String toString() {
return (digits(mostSigBits >> 32, 8) + "-" +
digits(mostSigBits >> 16, 4) + "-" +
digits(mostSigBits, 4) + "-" +
digits(leastSigBits >> 48, 4) + "-" +
digits(leastSigBits, 12));
}
/** Returns val represented by the specified number of hex digits. */
private static String digits(long val, int digits) {
long hi = 1L << (digits * 4);
return Long.toHexString(hi | (val & (hi - 1))).substring(1);
}
/**
* Returns a hash code for this {@code UUID}.
*
* @return A hash code value for this {@code UUID}
*/
public int hashCode() {
long hilo = mostSigBits ^ leastSigBits;
return ((int)(hilo >> 32)) ^ (int) hilo;
}
/**
* Compares this object to the specified object. The result is {@code
* true} if and only if the argument is not {@code null}, is a {@code UUID}
* object, has the same variant, and contains the same value, bit for bit,
* as this {@code UUID}.
*
* @param obj
* The object to be compared
*
* @return {@code true} if the objects are the same; {@code false}
* otherwise
*/
public boolean equals(Object obj) {
if ((null == obj) || (obj.getClass() != UUID.class))
return false;
UUID id = (UUID)obj;
return (mostSigBits == id.mostSigBits &&
leastSigBits == id.leastSigBits);
}
// Comparison Operations
/**
* Compares this UUID with the specified UUID.
* 对比该UUID和指定的UUID。
* 如果第一个UUID的UUID差异最大的字段更大，则两个UUID中的第一个大于第二个。
*
* <p> The first of two UUIDs is greater than the second if the most
* significant field in which the UUIDs differ is greater for the first
* UUID.
*
* @param val
* {@code UUID} to which this {@code UUID} is to be compared
*
* @return -1, 0 or 1 as this {@code UUID} is less than, equal to, or
* greater than {@code val}
*
*/
public int compareTo(UUID val) {
// The ordering is intentionally set up so that the UUIDs
// can simply be numerically compared as two numbers
return (this.mostSigBits < val.mostSigBits ? -1 :
(this.mostSigBits > val.mostSigBits ? 1 :
(this.leastSigBits < val.leastSigBits ? -1 :
(this.leastSigBits > val.leastSigBits ? 1 :
0))));
}
}
问题记录
