CRC32.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
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package java.util.zip;

import java.nio.ByteBuffer;
import java.util.Objects;

import sun.nio.ch.DirectBuffer;
import jdk.internal.HotSpotIntrinsicCandidate;

/**
 * A class that can be used to compute the CRC-32 of a data stream.
 *
 * <p> Passing a {@code null} argument to a method in this class will cause
 * a {@link NullPointerException} to be thrown.</p>
 *
 * @author David Connelly
 * @since 1.1
 */
// CRC32循环冗余校验的实现
public class CRC32 implements Checksum {
    
    private int crc;    // CRC-32校验和
    
    static {
        ZipUtils.loadLibrary();
    }
    
    /**
     * Creates a new CRC32 object.
     */
    public CRC32() {
    }
    
    /**
     * Updates the CRC-32 checksum with the specified byte (the low
     * eight bits of the argument b).
     */
    // 用指定的字节更新当前校验和
    @Override
    public void update(int b) {
        crc = update(crc, b);
    }
    
    /**
     * Updates the CRC-32 checksum with the specified array of bytes.
     *
     * @throws ArrayIndexOutOfBoundsException if {@code off} is negative, or {@code len} is negative, or
     *                                        {@code off+len} is negative or greater than the length of
     *                                        the array {@code b}.
     */
    // 用字节数组b中off处起的len个字节更新当前校验和
    @Override
    public void update(byte[] b, int off, int len) {
        if(b == null) {
            throw new NullPointerException();
        }
        
        if(off<0 || len<0 || off>b.length - len) {
            throw new ArrayIndexOutOfBoundsException();
        }
        
        crc = updateBytes(crc, b, off, len);
    }
    
    /**
     * Updates the CRC-32 checksum with the bytes from the specified buffer.
     *
     * The checksum is updated with the remaining bytes in the buffer, starting
     * at the buffer's position. Upon return, the buffer's position will be
     * updated to its limit; its limit will not have been changed.
     *
     * @since 1.8
     */
    // 用缓冲区buffer中的字节更新当前校验和
    @Override
    public void update(ByteBuffer buffer) {
        int pos = buffer.position();
        int limit = buffer.limit();
        
        assert (pos<=limit);
        
        int rem = limit - pos;
        if(rem<=0) {
            return;
        }
        
        if(buffer instanceof DirectBuffer) {
            crc = updateByteBuffer(crc, ((DirectBuffer) buffer).address(), pos, rem);
        } else if(buffer.hasArray()) {
            crc = updateBytes(crc, buffer.array(), pos + buffer.arrayOffset(), rem);
        } else {
            byte[] b = new byte[Math.min(buffer.remaining(), 4096)];
            while(buffer.hasRemaining()) {
                int length = Math.min(buffer.remaining(), b.length);
                buffer.get(b, 0, length);
                update(b, 0, length);
            }
        }
        
        buffer.position(limit);
    }
    
    /**
     * Returns CRC-32 value.
     */
    // 获取当前校验和
    @Override
    public long getValue() {
        return (long) crc & 0xffffffffL;
    }
    
    /**
     * Resets CRC-32 to initial value.
     */
    // 重置当前校验和
    @Override
    public void reset() {
        crc = 0;
    }
    
    // 用指定的字节更新当前校验和
    @HotSpotIntrinsicCandidate
    private static native int update(int crc, int b);
    
    // 用字节数组b中off处起的len个字节更新校验和crc
    private static int updateBytes(int crc, byte[] b, int off, int len) {
        updateBytesCheck(b, off, len);
        return updateBytes0(crc, b, off, len);
    }
    
    // 用地址addr处的缓冲区中off处起的len个字节更新校验和crc
    private static int updateByteBuffer(int crc, long addr, int off, int len) {
        updateByteBufferCheck(addr);
        return updateByteBuffer0(crc, addr, off, len);
    }
    
    // (本地实现)用字节数组b中off处起的len个字节更新校验和crc
    @HotSpotIntrinsicCandidate
    private static native int updateBytes0(int crc, byte[] b, int off, int len);
    
    // (本地实现)用地址addr处的缓冲区中off处起的len个字节更新校验和crc
    @HotSpotIntrinsicCandidate
    private static native int updateByteBuffer0(int crc, long addr, int off, int len);
    
    // 边界检查
    private static void updateBytesCheck(byte[] b, int off, int len) {
        if(len<=0) {
            return;  // not an error because updateBytesImpl won't execute if len <= 0
        }
        
        Objects.requireNonNull(b);
        
        if(off<0 || off >= b.length) {
            throw new ArrayIndexOutOfBoundsException(off);
        }
        
        int endIndex = off + len - 1;
        if(endIndex<0 || endIndex >= b.length) {
            throw new ArrayIndexOutOfBoundsException(endIndex);
        }
    }
    
    // 地址检查
    private static void updateByteBufferCheck(long addr) {
        // Performs only a null check because bounds checks are not easy to do on raw addresses.
        if(addr == 0L) {
            throw new NullPointerException();
        }
    }
    
}