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Source code analysis of GZIPOutputStream class

2022-08-01 18:55:00 【51CTO】

这是《水煮 JDK 源码》系列的第5篇文章,计划撰写100篇关于JDK源码相关的文章

GZIPOutputStream 类位于 java.util.zip 包下,继承于 DeflaterOutputStream 类,它实现了一个流式过滤器,主要用于以GZIPfile format to write compressed data,其UML类图如下：

GZIPOutputStream 类源码分析_java

类声明如下：

1、成员变量

GZIPOutputStream 定义了1个成员变量,如下：

CRC32 Is a computational data flow CRC-32 Checksum class,Mainly used to verify the integrity of compressed data.

2、构造函数

创建 GZIPOutputStream The compressed output streams are mainly4种方式,如下：

public GZIPOutputStream(OutputStream out) throws IOException {
    /** Create a new output stream with a buffer of default size,默认大小为 512,Simultaneous no synchronous refresh */
    this(out, 512, false);
}

public GZIPOutputStream(OutputStream out, boolean syncFlush)
    throws IOException
{
     /** Create a new output stream with a buffer of default size,默认大小为 512 */
    this(out, 512, syncFlush);
}

public GZIPOutputStream(OutputStream out, int size) throws IOException {
    /** Creates a new output stream with a buffer of the specified size,No synchronous refresh */
    this(out, size, false);
}

public GZIPOutputStream(OutputStream out, int size, boolean syncFlush)
    throws IOException
{
    /** Creates a new output stream with a buffer of the specified size */
    super(out, new Deflater(Deflater.DEFAULT_COMPRESSION, true),
          size,
          syncFlush);
    // use default compressor
    usesDefaultDeflater = true;
    // 写入 GZIP member header
    writeHeader();
    // 重置 CRC-32 校验
    crc.reset();
}

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GZIPOutputStream The class is mainly used for writingGZIPoutput stream of compressed data,Hence the need for a compressor Deflater.

3、写入数据方法

GZIPOutputStream The class provides a method to write data,其定义如下：

public synchronized void write(byte[] buf, int off, int len)
    throws IOException
{
    // 调用父类 DeflaterOutputStream 的 write() method to write byte array data
    super.write(buf, off, len);
    // update byte array data CRC32 校验和
    crc.update(buf, off, len);
}

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在创建 GZIPOutputStream When compressing the output stream,会使用 writeHeader() 方法写入GZIPmember header,So what header information will be written?？You can look at the definition of this method：

private void writeHeader() throws IOException {
    // 这个 out 是定义在父类 FilterOutputStream 中的成员变量
    out.write(new byte[] {
        (byte) GZIP_MAGIC,        // Magic number (short)
        (byte)(GZIP_MAGIC >> 8),  // Magic number (short)
        Deflater.DEFLATED,        // Compression method (CM)
        0,                        // Flags (FLG)
        0,                        // Modification time MTIME (int)
        0,                        // Modification time MTIME (int)
        0,                        // Modification time MTIME (int)
        0,                        // Modification time MTIME (int)
        0,                        // Extra flags (XFLG)
        0                         // Operating system (OS)
    });
}

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从代码实现可以看出,will write in total10个字节的头信息,Include header information magic number、Compression method and various flags, etc.,These header information also need to be checked one by one when decompressing.

Have write head information method,There will naturally be a way to write tail information,about writing tail information writeTrailer() 方法定义如下：

private void writeTrailer(byte[] buf, int offset) throws IOException {
    // 写入 CRC32 的 value 值,int 型数据占用 4个字节
    writeInt((int)crc.getValue(), buf, offset); // CRC-32 of uncompr. data
    // Total number of uncompressed bytes written
    writeInt(def.getTotalIn(), buf, offset + 4); // Number of uncompr. bytes
}

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上面调用的 writeInt() 方法定义如下：

private void writeInt(int i, byte[] buf, int offset) throws IOException {
    // 在写入 int 数据时,is written twice short 型数据
    writeShort(i & 0xffff, buf, offset);
    writeShort((i >> 16) & 0xffff, buf, offset + 2);
}

private void writeShort(int s, byte[] buf, int offset) throws IOException {
    // 而写入 short 数据时,write twice byte 型数据
    buf[offset] = (byte)(s & 0xff);
    buf[offset + 1] = (byte)((s >> 8) & 0xff);
}

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4、其他方法

When all compressed data is written to the output stream,This time you can call to complete finish() 方法,如下：

public void finish() throws IOException {
    // Determine whether the compressed data is complete,This is calling the compressor Deflater 的 finished() 方法
    // 如果没有完成,则调用 Deflater 的 finish() 完成方法
    if (!def.finished()) {
        def.finish();
        while (!def.finished()) {
            // 压缩 buf 数据
            int len = def.deflate(buf, 0, buf.length);
            // 如果 def 已经完成,But the length of the compressed data is less than minus the length of the tail information
            // write tail information
            if (def.finished() && len <= buf.length - TRAILER_SIZE) {
                // last deflater buffer. Fit trailer at the end
                writeTrailer(buf, len);
                len = len + TRAILER_SIZE;
                out.write(buf, 0, len);
                return;
            }
            // If the compressed data length is greater than 0,write directly to the output stream
            if (len > 0)
                out.write(buf, 0, len);
        }
        // if we can't fit the trailer at the end of the last
        // deflater buffer, we write it separately
        // Separate write tail information,The length of the tail information is 8个字节
        byte[] trailer = new byte[TRAILER_SIZE];
        writeTrailer(trailer, 0);
        // The tail information written to
        out.write(trailer);
    }
}

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5、测试应用

GZIPOutputStream Classes can be used to compress data,Then a sample code for compressing a string is given below：

package com.magic.test;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.zip.GZIPOutputStream;

public class GZIPOutputStreamTest {

    public static void main(String[] args) {
        String str = "0123456";
        try (ByteArrayOutputStream out = new ByteArrayOutputStream();
             GZIPOutputStream gzip = new GZIPOutputStream(out)) {

            // 将字符串 str Convert to byte array and write to gzip compressed output stream
            gzip.write(str.getBytes());

            // print compressed data
            System.out.println(out.toString());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

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