（代码篇）从基础文件IO说起虚拟内存，内存文件映射，零拷贝

FileInputStream in = new FileInputStream("D:\\java.txt"); 
in.read();

    public int read() throws IOException {
        return this.read0();
    }

    private int read0() throws IOException {
        Object arg0 = this.lock;
        synchronized (this.lock) {
           ...int arg2 = this.read(arg1, 0, 2);
           ...   
        }
    }

    public int read(char[] arg0, int arg1, int arg2) throws IOException {
        int arg3 = arg1;
        int arg4 = arg2;
        Object arg5 = this.lock;
        synchronized (this.lock) {
            this.ensureOpen();
            ...return arg6 + this.implRead(arg0, arg3, arg3 + arg4);
         　　...
        }
    }

FileInputStream in = new FileInputStream("D:\\java.txt"); 
BufferedInputStream intsmaze= new BufferedInputStream(in); 
intsmaze.read();

public class BufferedReader extends Reader {
    private Reader in;
    private char cb[];
    private int nChars, nextChar;
    private static int defaultCharBufferSize = 8192;
    public BufferedReader(Reader in, int sz) {
        this.in = in;
        cb = new char[sz];
        nextChar = nChars = 0;
    }
    public BufferedReader(Reader in) {
        this(in, defaultCharBufferSize);
    }
    public int read() throws IOException {
        synchronized (lock) {
            for (;;) {
                if (nextChar >= nChars) {
                    fill();
                }
                ......
                return cb[nextChar++];
            }
        }
    }
    private void fill() throws IOException {
        ......//判断逻辑
        int n;  
        n = in.read(cb, dst, cb.length - dst);//从这里可以看到，缓冲类中还是调用的FileReader的read(char cbuf[], int offset, int length)方法
        ......
    }

static XxxBuffer allocate(int capacity)//创建一个容量为capacity的XxxBuffer对象，内部创建子类对象为HeapXxxBuffer。

char[] myArray=new char[100];
CharBuffer charBuffer=CharBuffer.wrap(myArray);

CharBuffer charBuffer=CharBuffer.wrap(myArray,12,48);

        FileInputStream fis = new FileInputStream(file); 
        FileChannel channel = fis.getChannel();

        byte[] bytes = new byte[1024];
        ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
 
        // channel.read(ByteBuffer) 方法就类似于 inputstream.read(byte)
        // 每次read都将读取 1024个字节到ByteBuffer
        int len;
        while ((len = channel.read(byteBuffer)) != -1) {
            byteBuffer.flip();
        // 类似与InputStrean的read(byte[],offset,len)方法读取 
            byteBuffer.get(bytes, 0, len);

            byteBuffer.clear(); 
        }
        channel.close();
        fis.close();

 // 分配128MB直接内存
ByteBuffer bb = ByteBuffer.allocateDirect(1024 * 1024 * 128);

        File file=new File("D://develop/CodeWorkSpace/1.txt");
        FileInputStream fr = new FileInputStream(file);
        FileChannel fileChannel=fr.getChannel(); 
 //      map()将channel对应的全部或部分数据映射成MappedByteBuffer，第一个参数是映射模式，第二，第三参数就是用于将fileChannel哪些数据映射到buffer中。
 //      MappedByteBuffer mappedByteBuffer=fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, file.length());//将fileChannel全部数据映射到buffer       
        MappedByteBuffer mappedByteBuffer=fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, 20);
        CharBuffer cb=Charset.forName("UTF-8").newDecoder().decode(mappedByteBuffer);
        System.out.println(cb);
        mappedByteBuffer.clear();
        mappedByteBuffer=fileChannel.map(FileChannel.MapMode.READ_ONLY, 21, 20);
        cb=Charset.forName("UTF-8").newDecoder().decode(mappedByteBuffer);
        System.out.println("-------------------------");
        System.out.println(cb);
        mappedByteBuffer.clear();

/**
 * @author:YangLiu
 * @describe: 
 *            测试一:设置JVM参数-Xmx100m，运行异常，因为如果没设置-XX:MaxDirectMemorySize，则默认与-Xmx参数值相同
 *            ，分配128M直接内存超出限制范围。
 *            测试用例2：设置JVM参数-Xmx256m，运行正常，因为128M小于256M，属于范围内分配。
 *            测试用例3：设置JVM参数-Xmx256m
 *            -XX:MaxDirectMemorySize=100M，运行异常，分配的直接内存128M超过限定的100M。
 *            测试用例4：设置JVM参数
 *            -Xmx768m，运行程序观察内存使用变化，会发现clean()后内存马上下降，说明使用clean()方法能有效及时回收直接缓存。
 */
public class DirectByteBufferTest {
    public static void main(String[] args) throws InterruptedException {
        // 分配128MB直接内存
        ByteBuffer bb = ByteBuffer.allocateDirect(1024 * 1024 * 128);

        TimeUnit.SECONDS.sleep(20);
        System.out.println("intsmaze");
        // 清除直接缓存
        ((DirectBuffer) bb).cleaner().clean();
        System.out.println("intsmaze");
    }
}