最近在查一个内存泄露问题的时候查到的，总所周知-XX:MaxDirectMemorySize可以设置java堆外内存的峰值，但是具体是在哪里限制的呢

java.nio.DirectByteBuffer用于生成一块cap大小的堆外内存

java.nio.Bits

// These methods should be called whenever direct memory is allocated or
    // freed.  They allow the user to control the amount of direct memory
    // which a process may access.  All sizes are specified in bytes.
    static void reserveMemory(long size, int cap) {
        synchronized (Bits.class) {
            if (!memoryLimitSet && VM.isBooted()) {
                maxMemory = VM.maxDirectMemory();
                memoryLimitSet = true;
            }
            // -XX:MaxDirectMemorySize limits the total capacity rather than the
            // actual memory usage, which will differ when buffers are page
            // aligned.
            if (cap <= maxMemory - totalCapacity) { //这里操作了totalCapacity，注意是加锁的哦
                reservedMemory += size;
                totalCapacity += cap;
                count++;
                return;
            }
        }

        System.gc();
        try {
            Thread.sleep(100);//其实jvm也用了很low的sleep下么
        } catch (InterruptedException x) {
            // Restore interrupt status
            Thread.currentThread().interrupt();
        }
        synchronized (Bits.class) {
            if (totalCapacity + cap > maxMemory) //这里超过就异常了
                throw new OutOfMemoryError("Direct buffer memory");
            reservedMemory += size;
            totalCapacity += cap;
            count++;
        }

    }

 
 补充一下，计算totalCapacity的时候加了同步锁，所以申请堆外内存的时候也限制了服务性能。