如何创建 DirectByteBuffer

通过 ByteBuffer.allocateDirect(int capacity) 方法创建

public static ByteBuffer allocateDirect(int capacity) {
    return new DirectByteBuffer(capacity);
}

源码分析

分析 DirectByteBuffer 构造函数，创建 DirectByteBuffer 可分为三步：

1. 计算需要分配的内存大小
2. 分配内存，保存内存地址
3. 创建内存回收器

DirectByteBuffer(int cap) {                   // package-private

    super(-1, 0, cap, cap);
    // 是否需要内存分页对齐
    boolean pa = VM.isDirectMemoryPageAligned(); 
    // 获取内存分页大小
    int ps = Bits.pageSize();
    // 计算需要分配的内存大小：如果需要分页对齐则加上1个分页
    long size = Math.max(1L, (long)cap + (pa ? ps : 0));
    Bits.reserveMemory(size, cap);

    long base = 0;
    try {
        // 分配内存
        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;
    }
    // 创建内存回收器
    cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
    att = null;
}