UUID.java

/*
 * Copyright (c) 2003, 2014, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */

package java.util;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import jdk.internal.misc.JavaLangAccess;
import jdk.internal.misc.SharedSecrets;

/**
 * A class that represents an immutable universally unique identifier (UUID).
 * A UUID represents a 128-bit value.
 *
 * <p> There exist different variants of these global identifiers.  The methods
 * of this class are for manipulating the Leach-Salz variant, although the
 * constructors allow the creation of any variant of UUID (described below).
 *
 * <p> The layout of a variant 2 (Leach-Salz) UUID is as follows:
 *
 * The most significant long consists of the following unsigned fields:
 * <pre>
 * 0xFFFFFFFF00000000 time_low
 * 0x00000000FFFF0000 time_mid
 * 0x000000000000F000 version
 * 0x0000000000000FFF time_hi
 * </pre>
 * The least significant long consists of the following unsigned fields:
 * <pre>
 * 0xC000000000000000 variant
 * 0x3FFF000000000000 clock_seq
 * 0x0000FFFFFFFFFFFF node
 * </pre>
 *
 * <p> The variant field contains a value which identifies the layout of the
 * {@code UUID}.  The bit layout described above is valid only for a {@code
 * UUID} with a variant value of 2, which indicates the Leach-Salz variant.
 *
 * <p> The version field holds a value that describes the type of this {@code
 * UUID}.  There are four different basic types of UUIDs: time-based, DCE
 * security, name-based, and randomly generated UUIDs.  These types have a
 * version value of 1, 2, 3 and 4, respectively.
 *
 * <p> For more information including algorithms used to create {@code UUID}s,
 * see <a href="http://www.ietf.org/rfc/rfc4122.txt"> <i>RFC&nbsp;4122: A
 * Universally Unique IDentifier (UUID) URN Namespace</i></a>, section 4.2
 * &quot;Algorithms for Creating a Time-Based UUID&quot;.
 *
 * @since 1.5
 */
/*
 * UUID的意思是：通用唯一识别码。该类用于生成唯一的标识
 *
 * 更多生成方式参见：https://mvnrepository.com/artifact/com.fasterxml.uuid/java-uuid-generator
 */
public final class UUID implements java.io.Serializable, Comparable<UUID> {
    
    /**
     * Explicit serialVersionUID for interoperability.
     */
    private static final long serialVersionUID = -4856846361193249489L;
    
    private static final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
    
    /**
     * 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;
    
    
    
    /*▼ 构造方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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}.
     *
     * @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;
    }
    
    /**
     * 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;
    }
    
    /*▲ 构造方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 生成UUID ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Static factory to retrieve a type 4 (pseudo randomly generated) UUID.
     *
     * The {@code UUID} is generated using a cryptographically strong pseudo m number generator.
     *
     * @return A randomly generated {@code UUID}
     */
    // 生成第4类UUID，参见version()
    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.
     *
     * @param name A byte array to be used to construct a {@code UUID}
     *
     * @return A {@code UUID} generated from the specified array
     */
    // 生成第3类UUID，参见version()。（name一致时，UUID也一致）
    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);
    }
    
    /*▲ 生成UUID ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 版本/变体 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * The version number associated with this {@code UUID}.
     * The version number describes how this {@code UUID} was generated.
     *
     * 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}
     */
    /*
     * UUID版本类型，描述了UUID的生成依据
     *
     * 1    Time-based UUID
     * 2    DCE security UUID
     * 3    Name-based UUID
     * 4    Randomly generated 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}.
     *
     * 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&nbsp;RFC&nbsp;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}
     */
    // UUID变体类型，描述了UUID的布局结构
    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));
    }
    
    /*▲ 版本/变体 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 字符串化/逆字符串化 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a {@code String} object representing this {@code UUID}.
     *
     * <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}
     */
    // 字符串化
    public String toString() {
        return jla.fastUUID(leastSigBits, mostSigBits);
    }
    
    /**
     * Creates a {@code UUID} from the string standard representation as described in the {@link #toString} method.
     *
     * @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 #toString}
     */
    // 逆字符串化
    public static UUID fromString(String name) {
        int len = name.length();
        if(len>36) {
            throw new IllegalArgumentException("UUID string too large");
        }
        
        int dash1 = name.indexOf('-');
        int dash2 = name.indexOf('-', dash1 + 1);
        int dash3 = name.indexOf('-', dash2 + 1);
        int dash4 = name.indexOf('-', dash3 + 1);
        int dash5 = name.indexOf('-', dash4 + 1);
        
        // For any valid input, dash1 through dash4 will be positive and dash5
        // negative, but it's enough to check dash4 and dash5:
        // - if dash1 is -1, dash4 will be -1
        // - if dash1 is positive but dash2 is -1, dash4 will be -1
        // - if dash1 and dash2 is positive, dash3 will be -1, dash4 will be
        //   positive, but so will dash5
        if(dash4<0 || dash5 >= 0) {
            throw new IllegalArgumentException("Invalid UUID string: " + name);
        }
        
        long mostSigBits = Long.parseLong(name, 0, dash1, 16) & 0xffffffffL;
        mostSigBits <<= 16;
        mostSigBits |= Long.parseLong(name, dash1 + 1, dash2, 16) & 0xffffL;
        mostSigBits <<= 16;
        mostSigBits |= Long.parseLong(name, dash2 + 1, dash3, 16) & 0xffffL;
        long leastSigBits = Long.parseLong(name, dash3 + 1, dash4, 16) & 0xffffL;
        leastSigBits <<= 48;
        leastSigBits |= Long.parseLong(name, dash4 + 1, len, 16) & 0xffffffffffffL;
        
        return new UUID(mostSigBits, leastSigBits);
    }
    
    /*▲ 字符串化/逆字符串化 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Returns the least significant 64 bits of this UUID's 128 bit value.
     *
     * @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.
     *
     * @return The most significant 64 bits of this UUID's 128 bit value
     */
    public long getMostSignificantBits() {
        return mostSigBits;
    }
    
    /**
     * 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.
     *
     * @return The timestamp of this {@code UUID}.
     *
     * @throws UnsupportedOperationException If this UUID is not a version 1 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.
     *
     * @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.
     *
     * @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;
    }
    
    /**
     * 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);
    }
    
    /**
     * Compares this UUID with the specified UUID.
     *
     * <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))));
    }
    
    /**
     * The random number generator used by this class to create random
     * based UUIDs. In a holder class to defer initialization until needed.
     */
    // 持有安全的随机数生成器
    private static class Holder {
        static final SecureRandom numberGenerator = new SecureRandom();
    }
}