Java ByteBuffer性能问题

Java ByteBuffer性能问题

我相信你只是在做微优化，这 可能只是 无关紧要 （www.codinghorror.com）

。

下面是一个具有较大缓冲区并删除了多余

seek

/

setPosition

调用的版本。

• 当我启用“本机字节排序”时（如果机器使用其他“ endian”约定，这实际上是不安全的）：

mmap: 1.358bytebuffer: 0.922regular i/o: 1.387

• 当我注释掉order语句并使用默认的big-endian排序时：

mmap: 1.336bytebuffer: 1.62regular i/o: 1.467

• 您的原始代码：

mmap: 3.262bytebuffer: 106.676regular i/o: 90.903

这是代码：

import java.io.File;import java.io.IOException;import java.io.RandomAccessFile;import java.nio.ByteBuffer;import java.nio.ByteOrder;import java.nio.channels.FileChannel;import java.nio.channels.FileChannel.MapMode;import java.nio.MappedByteBuffer;class Testbb2 {        static final int BUFFSIZE = 0x800 * 8; // 8192    static final int DATASIZE = 0x8000 * BUFFSIZE;    static public long byteArrayToLong(byte [] in, int offset) {        return ((((((((long)(in[offset + 0] & 0xff) << 8) | (long)(in[offset + 1] & 0xff)) << 8 | (long)(in[offset + 2] & 0xff)) << 8 | (long)(in[offset + 3] & 0xff)) << 8 | (long)(in[offset + 4] & 0xff)) << 8 | (long)(in[offset + 5] & 0xff)) << 8 | (long)(in[offset + 6] & 0xff)) << 8 | (long)(in[offset + 7] & 0xff);    }    public static void main(String [] args) throws IOException {        long start;        RandomAccessFile fileHandle;        FileChannel fileChannel;        // Sanity check - this way the convert-to-long loops don't need extra bookkeeping like BUFFSIZE / 8.        if ((DATASIZE % BUFFSIZE) > 0 || (DATASIZE % 8) > 0) { throw new IllegalStateException("DATASIZE should be a multiple of 8 and BUFFSIZE!");        }        int pos;        int nDone;        // create file        File testFile = new File("file.dat");        fileHandle = new RandomAccessFile("file.dat", "rw");        if (testFile.exists() && testFile.length() >= DATASIZE) { System.out.println("File exists");        } else { testFile.delete(); System.out.println("Preparing file"); byte [] buffer = new byte[BUFFSIZE]; pos = 0; nDone = 0; while (pos < DATASIZE) {     fileHandle.write(buffer);     pos += buffer.length; } System.out.println("File prepared");        }         fileChannel = fileHandle.getChannel();        // mmap()        MappedByteBuffer mbb = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, DATASIZE);        byte [] buffer1 = new byte[BUFFSIZE];        mbb.position(0);        start = System.currentTimeMillis();        pos = 0;        while (pos < DATASIZE) { mbb.get(buffer1, 0, BUFFSIZE); // This assumes BUFFSIZE is a multiple of 8. for (int i = 0; i < BUFFSIZE; i += 8) {     long dummy = byteArrayToLong(buffer1, i); } pos += BUFFSIZE;        }        System.out.println("mmap: " + (System.currentTimeMillis() - start) / 1000.0);        // bytebuffer        ByteBuffer buffer2 = ByteBuffer.allocateDirect(BUFFSIZE);//        buffer2.order(ByteOrder.nativeOrder());        buffer2.order();        fileChannel.position(0);        start = System.currentTimeMillis();        pos = 0;        nDone = 0;        while (pos < DATASIZE) { buffer2.rewind(); fileChannel.read(buffer2); buffer2.rewind();   // need to rewind it to be able to use it // This assumes BUFFSIZE is a multiple of 8. for (int i = 0; i < BUFFSIZE; i += 8) {     long dummy = buffer2.getLong(); } pos += BUFFSIZE;        }        System.out.println("bytebuffer: " + (System.currentTimeMillis() - start) / 1000.0);        // regular i/o        fileHandle.seek(0);        byte [] buffer3 = new byte[BUFFSIZE];        start = System.currentTimeMillis();        pos = 0;        while (pos < DATASIZE && nDone != -1) { nDone = 0; while (nDone != -1  && nDone < BUFFSIZE) {     nDone = fileHandle.read(buffer3, nDone, BUFFSIZE - nDone); } // This assumes BUFFSIZE is a multiple of 8. for (int i = 0; i < BUFFSIZE; i += 8) {     long dummy = byteArrayToLong(buffer3, i); } pos += nDone;        }        System.out.println("regular i/o: " + (System.currentTimeMillis() - start) / 1000.0);    }}