CheckedInputStream.java

/*
 * Copyright (c) 1996, 2006, 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.zip;

import java.io.FilterInputStream;
import java.io.InputStream;
import java.io.IOException;

/**
 * An input stream that also maintains a checksum of the data being read.
 * The checksum can then be used to verify the integrity of the input data.
 *
 * @author David Connelly
 * @see Checksum
 * @since 1.1
 */
// 包含数据校验器的输入流，可以对读自输入流的数据进行数据校验
public class CheckedInputStream extends FilterInputStream {
    private Checksum cksum;     // 数据校验
    
    /**
     * Creates an input stream using the specified Checksum.
     *
     * @param in    the input stream
     * @param cksum the Checksum
     */
    public CheckedInputStream(InputStream in, Checksum cksum) {
        super(in);
        this.cksum = cksum;
    }
    
    /**
     * Reads a byte. Will block if no input is available.
     *
     * @return the byte read, or -1 if the end of the stream is reached.
     *
     * @throws IOException if an I/O error has occurred
     */
    // 读取一个字节，返回读到的字节
    public int read() throws IOException {
        int b = in.read();
        if(b != -1) {
            cksum.update(b);
        }
        return b;
    }
    
    /**
     * Reads into an array of bytes. If <code>len</code> is not zero, the method
     * blocks until some input is available; otherwise, no
     * bytes are read and <code>0</code> is returned.
     *
     * @param buf the buffer into which the data is read
     * @param off the start offset in the destination array <code>b</code>
     * @param len the maximum number of bytes read
     *
     * @return the actual number of bytes read, or -1 if the end
     * of the stream is reached.
     *
     * @throws NullPointerException      If <code>buf</code> is <code>null</code>.
     * @throws IndexOutOfBoundsException If <code>off</code> is negative,
     *                                   <code>len</code> is negative, or <code>len</code> is greater than
     *                                   <code>buf.length - off</code>
     * @throws IOException               if an I/O error has occurred
     */
    // 读取len个字节，读到的字节存入buf的off处，返回实际读到的字节数
    public int read(byte[] buf, int off, int len) throws IOException {
        len = in.read(buf, off, len);
        if(len != -1) {
            cksum.update(buf, off, len);
        }
        
        return len;
    }
    
    /**
     * Skips specified number of bytes of input.
     *
     * @param n the number of bytes to skip
     *
     * @return the actual number of bytes skipped
     *
     * @throws IOException if an I/O error has occurred
     */
    // 跳过n个字节
    public long skip(long n) throws IOException {
        byte[] buf = new byte[512];
        long total = 0;
        while(total<n) {
            long len = n - total;
            len = read(buf, 0, len<buf.length ? (int) len : buf.length);
            if(len == -1) {
                return total;
            }
            total += len;
        }
        return total;
    }
    
    /**
     * Returns the Checksum for this input stream.
     *
     * @return the Checksum value
     */
    // 返回数据校验和
    public Checksum getChecksum() {
        return cksum;
    }
}