Java NIO和BIO比较

Java NIO和BIO比较

学过计算机组成原理就知道在IO系统中直接内存访问DMA方式相比程序中断方式访问方式，进一步提高了CPU利用率，这里从Java程序通过对读写一个文件来进行一个验证。

public class TestChanel {
    
    
    public static void main(String[] args) throws IOException {
    
    
        long start = System.currentTimeMillis();
        FileChannel reader = FileChannel.open(Path.of("Tomcat内核设计剖析.pdf"));
        FileChannel writer = FileChannel.open(Path.of("Tomcat内核设计剖析ChannelCopy.pdf"), StandardOpenOption.CREATE, StandardOpenOption.WRITE);
        ByteBuffer allocate = ByteBuffer.allocate(1024*1024);
        int read = reader.read(allocate);
        while (read != 0) {
    
    
            writer.write(allocate);
            read = reader.read(allocate);
        }

        System.out.println("The channel spend time: " + (System.currentTimeMillis() - start));
        reader.close();
        writer.close();
        start = System.currentTimeMillis();
        BufferedInputStream bufferedInputStream = new BufferedInputStream(new FileInputStream("Tomcat内核设计剖析.pdf"));
        BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(new FileOutputStream("Tomcat内核设计剖析BufferCopy.pdf"));
        byte[] bytes = new byte[1024*1024];
        read = bufferedInputStream.read(bytes);
        while (read != -1) {
    
    
            bufferedOutputStream.write(bytes);
            read = bufferedInputStream.read(bytes);
        }
        System.out.println("The buffer spend time: " + (System.currentTimeMillis() - start));
        bufferedInputStream.close();
        bufferedOutputStream.close();
    }
}

输出结果为:

The channel spend time: 5
The buffer spend time: 189

这结果确实相差很多，可见NIO读写效率相对传统BIO来说快了很多

NIO读写对象存储在虚拟机中的直接内存，可以使用-XX:MaxDirectMemorySize=Size参数来限制其大小。

Exception in thread "main" java.lang.OutOfMemoryError: Cannot reserve 1048576 bytes of direct buffer memory (allocated: 8192, limit: 1047552)
	at java.base/java.nio.Bits.reserveMemory(Bits.java:178)
	at java.base/java.nio.DirectByteBuffer.<init>(DirectByteBuffer.java:121)
	at java.base/java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:332)
	at java.base/sun.nio.ch.Util.getTemporaryDirectBuffer(Util.java:243)
	at java.base/sun.nio.ch.IOUtil.read(IOUtil.java:293)
	at java.base/sun.nio.ch.IOUtil.read(IOUtil.java:273)
	at java.base/sun.nio.ch.FileChannelImpl.read(FileChannelImpl.java:232)
	at study.java.nio.TestChanel.main(TestChanel.java:36)

对于这里需要分配1024KB，但如果只分配1023KB就会抛出OutOfMemoryError异常。