NIO基础之Buffer

public class BufferTest {
    public static void main(String[] args) throws IOException {
        ByteBufferTest();
    }
    private static void  ByteBufferTest(){
        //分配新的byte缓冲区，参数为缓冲区容量
        //新缓冲区的当前位置将为零，其界限(限制位置)将为其容量,它将具有一个底层实现数组，其数组偏移量将为零。
        ByteBuffer byteBuffer=ByteBuffer.allocate(10);
        output("初始化缓冲区：",byteBuffer);
        for(int i=0;i<byteBuffer.capacity()-1;i++){
            byteBuffer.put(Byte.parseByte(new SecureRandom().nextInt(20)+""));
        }
        output("写入缓冲区9个byte：",byteBuffer);
        byteBuffer.flip();
        output("使用flip重置元素位置：",byteBuffer);
        while (byteBuffer.hasRemaining()){
            System.out.print(byteBuffer.get()+"|");
        }
        System.out.print("\n");
        output("使用get读取元素：",byteBuffer);
        byteBuffer.clear();
        output("恢复初始化态clear：",byteBuffer);

    }
    private static void output(String step, Buffer buffer) {
        System.out.println(step + " : ");
        System.out.print("capacity: " + buffer.capacity() + ", ");
        System.out.print("position: " + buffer.position() + ", ");
        System.out.println("limit: " + buffer.limit());
        System.out.println("mark: " + buffer.mark());
        System.out.println();
    }
}
初始化缓冲区： : 
capacity: 10, position: 0, limit: 10
mark: java.nio.HeapByteBuffer[pos=0 lim=10 cap=10]

写入缓冲区9个byte： : 
capacity: 10, position: 9, limit: 10
mark: java.nio.HeapByteBuffer[pos=9 lim=10 cap=10]

使用flip重置元素位置： : 
capacity: 10, position: 0, limit: 9
mark: java.nio.HeapByteBuffer[pos=0 lim=9 cap=10]

读取元素：1|读取元素：16|读取元素：12|读取元素：0|读取元素：17|读取元素：5|读取元素：4|读取元素：13|读取元素：18|
使用get读取元素后： : 
capacity: 10, position: 9, limit: 9
mark: java.nio.HeapByteBuffer[pos=9 lim=9 cap=10]

恢复初始化态clear： : 
capacity: 10, position: 0, limit: 10
mark: java.nio.HeapByteBuffer[pos=0 lim=10 cap=10]

buffer.position( 3 );
buffer.limit( 7 );
ByteBuffer slice = buffer.slice();

 DirectByteBuffer(int cap) {                   // package-private
        super(-1, 0, cap, cap);
        boolean pa = VM.isDirectMemoryPageAligned();
        int ps = Bits.pageSize();
        long size = Math.max(1L, (long)cap + (pa ? ps : 0));
        // 保留总分配内存(按页分配)的大小和实际内存的大小
        Bits.reserveMemory(size, cap);
        long base = 0;
        try {
             // 通过unsafe.allocateMemory分配堆外内存，并返回堆外内存的基地址
            base = unsafe.allocateMemory(size);
        } catch (OutOfMemoryError x) {
            Bits.unreserveMemory(size, cap);
            throw x;
        }
        unsafe.setMemory(base, size, (byte) 0);
        if (pa && (base % ps != 0)) {
            // Round up to page boundary
            address = base + ps - (base & (ps - 1));
        } else {
            address = base;
        }
         // 构建Cleaner对象用于跟踪DirectByteBuffer对象的垃圾回收，以实现当DirectByteBuffer被垃圾回收时，堆外内存也会被释放
        cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
        att = null;
    }

 // Used only by direct buffers
    // NOTE: hoisted here for speed in JNI GetDirectBufferAddress
    //address就是堆外内存创建好后返回给JVM的地址,JVM内存需要维护的只是DirectByteBuffer对象，而具体数据的管理是由操作系统来管理的
    long address;

private static void directByteBufferTest()throws IOException{
        long start=System.currentTimeMillis();
        FileInputStream is=new FileInputStream("F:\\logs\\1g.rar");
        FileOutputStream fos=new FileOutputStream("F:\\logs\\2g.rar");
        FileChannel fcIs,fcOut;
        fcIs=is.getChannel();
        fcOut=fos.getChannel();
        ByteBuffer directByteBuffer= ByteBuffer.allocateDirect(2048);
        while (fcIs.read(directByteBuffer)!=-1){
            directByteBuffer.flip();
            fcOut.write(directByteBuffer);
            directByteBuffer.clear();
        }
        is.close();
        fos.close();
        long end=System.currentTimeMillis();
        System.out.println("DirectByteBuffer需要时间："+(end-start));
    }
    private static void heapByteBufferTest()throws IOException{
        long start=System.currentTimeMillis();
        FileInputStream is=new FileInputStream("F:\\logs\\1g.rar");
        FileOutputStream fos=new FileOutputStream("F:\\logs\\3g.rar");
        FileChannel fcIs,fcOut;
        fcIs=is.getChannel();
        fcOut=fos.getChannel();
        ByteBuffer directByteBuffer= ByteBuffer.allocate(2048);
        while (fcIs.read(directByteBuffer)!=-1){
            directByteBuffer.flip();
            fcOut.write(directByteBuffer);
            directByteBuffer.clear();
        }
        is.close();
        fos.close();
        long end=System.currentTimeMillis();
        System.out.println("HeapByteBuffer需要时间："+(end-start));
    }

private static void mappedOutFile()throws IOException{
        String str="I Love MappedByteBuffer";
        RandomAccessFile raf = new RandomAccessFile( filePath, "rw" );
        FileChannel fc = raf.getChannel();
        byte [] msg=str.getBytes("UTF-8");
        MappedByteBuffer mbb = fc.map( FileChannel.MapMode.READ_WRITE, 0, msg.length);
        mbb.put(msg);
        fc.write(mbb);
        raf.close();
    }