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Source code analysis of GZIPInputStream class

2022-07-31 09:48:00 【51CTO】

这是《水煮 JDK 源码》系列的第4篇文章,计划撰写100篇关于JDK源码相关的文章

GZIPInputStream 类位于 java.util.zip 包下,继承于 InflaterInputStream 类,It implements a streaming filter,主要用于读取GZIPCompressed data in file format,其UML类图如下：

GZIPInputStream 类源码分析_java

类声明如下：

1、成员变量

GZIPInputStream 定义了3个成员变量,分别如下：

/** CRC-32 for uncompressed data */
protected CRC32 crc = new CRC32();
/** Represents the end state of the input stream */
protected boolean eos;
/** The state of whether the input stream is closed */
private boolean closed = false;

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2、构造函数

创建 GZIPInputStream There are two main ways to compress the input stream：

/** Create a new input stream with a buffer of default size */
public GZIPInputStream(InputStream in) throws IOException {
    // 默认缓冲区大小为512
    this(in, 512);
}

/** Creates a new input stream with a buffer of the specified size */
public GZIPInputStream(InputStream in, int size) throws IOException {
    // 调用父类 InflaterInputStream 的构造函数
    super(in, new Inflater(true), size);
    // 设置父类 InflaterInputStream 的 usesDefaultInflater
    // Indicates to use the default decompressor
    usesDefaultInflater = true;
    // 读取 GZIP member header information,and returns the total number of bytes of header information
    readHeader(in);
}

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由于 GZIPInputStream is due to reading the input stream of compressed data,Therefore a decompressor is required Inflater.

3、读取数据方法

GZIPInputStream 主要提供了1A method for reading stream data,如下：

public int read(byte[] buf, int off, int len) throws IOException {
    // Before officially reading the stream data,Make sure the stream is not closed
    ensureOpen();
    // If the end of the stream has been reached,Indicates that there is no more readable data,直接返回 -1
    if (eos) {
        return -1;
    }
    // 调用父类 InflaterInputStream 的 read() 方法读取数据
    int n = super.read(buf, off, len);
    // If the actual read data is  -1,Description No readable data
    if (n == -1) {
        // 读取 GZIP member tail information,判断是否读取到了 eos
        // 如果是,则将 eos 置为 true,Indicates that the tail has been read
        if (readTrailer())
            eos = true;
        else
            // 否则,Continue to call this method to read
            return this.read(buf, off, len);
    } else {
        // If data is read,is updated with the specified byte data CRC32 的校验和
        crc.update(buf, off, n);
    }
    // 返回实际读取的字节数
    return n;
}

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在读取数据之前,Need to first check if the stream is closed,if the stream has been closed,The description is unreadable,ensureOpen() The method is for this purpose,其定义如下：

private void ensureOpen() throws IOException {
    if (closed) {
        // if the stream is closed,直接抛出 IOException 异常
        throw new IOException("Stream closed");
    }
}

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在创建 GZIPInputStream when entering a stream,需要去读取 GZIP member header information,readHeader() 方法定义如下：

/** GZIP head magic number */
public final static int GZIP_MAGIC = 0x8b1f;

/** 文件头标识 */
private final static int FTEXT      = 1;    // Extra text
private final static int FHCRC      = 2;    // Header CRC
private final static int FEXTRA     = 4;    // Extra field
private final static int FNAME      = 8;    // File name
private final static int FCOMMENT   = 16;   // File comment

private int readHeader(InputStream this_in) throws IOException {
    // 创建 CheckedInputStream,Checksum for maintaining data
    CheckedInputStream in = new CheckedInputStream(this_in, crc);
    // 重置 CRC32 校验
    crc.reset();
    // Check the head magic number,判断是否为 GZIP 格式
    if (readUShort(in) != GZIP_MAGIC) {
        throw new ZipException("Not in GZIP format");
    }
    // Check the compression method,Determines whether it is a supported compression method
    if (readUByte(in) != 8) {
        throw new ZipException("Unsupported compression method");
    }
    // 读取标识
    int flg = readUByte(in);
    // Skip MTIME, XFL, and OS fields
    // Skip special fields
    skipBytes(in, 6);
    int n = 2 + 2 + 6;
    // Skip optional extra field
    if ((flg & FEXTRA) == FEXTRA) {
        int m = readUShort(in);
        skipBytes(in, m);
        n += m + 2;
    }
    // Skip optional file name
    if ((flg & FNAME) == FNAME) {
        do {
            n++;
        } while (readUByte(in) != 0);
    }
    // Skip optional file comment
    if ((flg & FCOMMENT) == FCOMMENT) {
        do {
            n++;
        } while (readUByte(in) != 0);
    }
    // Check optional header CRC
    if ((flg & FHCRC) == FHCRC) {
        int v = (int)crc.getValue() & 0xffff;
        if (readUShort(in) != v) {
            throw new ZipException("Corrupt GZIP header");
        }
        n += 2;
    }
    // 重置 CRC32 校验
    crc.reset();
    // Returns the total number of bytes of header information
    return n;
}

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在读取数据的时候,需要读取GZIPtail information,And use this to judge whether the reading has ended,readTrailer() 方法如下：

private boolean readTrailer() throws IOException {
    InputStream in = this.in;
    // Gets the total remaining readable length in bytes,调用的是 Inflater 的 getRemaining 方法
    int n = inf.getRemaining();
    // If the number of readable bytes is greater than 0
    if (n > 0) {
        // Create a sequence input stream
        in = new SequenceInputStream(
            new ByteArrayInputStream(buf, len - n, n),
            new FilterInputStream(in) {
                public void close() throws IOException {}
            });
    }
    // Uses left-to-right evaluation order
    if ((readUInt(in) != crc.getValue()) ||
        // rfc1952; ISIZE is the input size modulo 2^32
        (readUInt(in) != (inf.getBytesWritten() & 0xffffffffL)))
        throw new ZipException("Corrupt GZIP trailer");

    // If there are more bytes available in "in" or
    // the leftover in the "inf" is > 26 bytes:
    // this.trailer(8) + next.header.min(10) + next.trailer(8)
    // try concatenated case
    if (this.in.available() > 0 || n > 26) {
        int m = 8;                  // this.trailer
        try {
            m += readHeader(in);    // next.header
        } catch (IOException ze) {
            return true;  // ignore any malformed, do nothing
        }
        inf.reset();
        if (n > m)
            inf.setInput(buf, len - n + m, n - m);
        return false;
    }
    return true;
}

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No matter when reading header information or tail information,will read the specified length of the identification bit,比如 readUInt、readUShort、readUByte 方法,They are used to read unsigned integers, respectively、无符号短整型、Unsigned byte data,其定义如下：

private long readUInt(InputStream in) throws IOException {
    long s = readUShort(in);
    return ((long)readUShort(in) << 16) | s;
}

private int readUShort(InputStream in) throws IOException {
    int b = readUByte(in);
    return (readUByte(in) << 8) | b;
}

private int readUByte(InputStream in) throws IOException {
    // 读取字节数据,Get the total number of bytes read
    int b = in.read();
    // If the total bytes are  -1,说明已经读完了
    if (b == -1) {
        throw new EOFException();
    }
    if (b < -1 || b > 255) {
        // Report on this.in, not argument in; see read{Header, Trailer}.
        throw new IOException(this.in.getClass().getName()
                              + ".read() returned value out of range -1..255: " + b);
    }
    return b;
}

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4、其他方法

When reading compressed data streams,It is also possible to skip a specified number of bytes,其方法定义如下：

private byte[] tmpbuf = new byte[128];
/** Skips bytes of data of the specified length in the input stream,该方法是阻塞的,until all bytes are skipped */
private void skipBytes(InputStream in, int n) throws IOException {
    while (n > 0) {
        int len = in.read(tmpbuf, 0, n < tmpbuf.length ? n : tmpbuf.length);
        if (len == -1) {
            throw new EOFException();
        }
        n -= len;
    }
}

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GZIPInputStream 的 close() 方法如下：

public void close() throws IOException {
    // First determine whether the input stream is closed
    if (!closed) {
        // 如果没有关闭,调用父类的 close method to close the input stream
        super.close();
        // 同时设置 eos 为 ture,Indicates that the input stream has ended
        eos = true;
        // Modify the closed state
        closed = true;
    }
}

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