java中Unsafe的使用讲解

//1.最简单的使用方式是基于反射获取Unsafe实例
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
Unsafe unsafe = (Unsafe) f.get(null);

private static Unsafe unsafe = null;
private static Field getUnsafe = null; 
static {
    try {
        getUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
        getUnsafe.setAccessible(true);
        unsafe = (Unsafe) getUnsafe.get(null);
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
}

String[] strings = new String[]{"1", "2", "3"};
long i = unsafe.arrayBaseOffset(String[].class);
System.out.println("string[] base offset is :" + i);
 
//every index scale
long scale = unsafe.arrayIndexScale(String[].class);
System.out.println("string[] index scale is " + scale);
 
//print first string in strings[]
System.out.println("first element is :" + unsafe.getObject(strings, i));
 
//set 100 to first string
unsafe.putObject(strings, i + scale * 0, "100");
 
//print first string in strings[] again
System.out.println("after set ,first element is :" + unsafe.getObject(strings, i + scale * 0));

//调用allocateInstance函数避免了在我们不需要构造函数的时候却调用它
Data data = (Data) unsafe.allocateInstance(Data.class);
data.setId(1L);
data.setName("unsafe");
System.out.println(data);
 
//返回成员属性在内存中的地址相对于对象内存地址的偏移量
Field nameField = Data.class.getDeclaredField("name");
long fieldOffset = unsafe.objectFieldOffset(nameField);
//putLong，putInt，putDouble，putChar，putObject等方法，直接修改内存数据（可以越过访问权限）
unsafe.putObject(data,fieldOffset,"这是新的值");
System.out.println(data.getName());
  

File file = new File("C:\\workspace\\idea2\\disruptor\\target\\classes\\com\\onyx\\distruptor\\test\\Data.class");
FileInputStream input = new FileInputStream(file);
byte[] content = new byte[(int)file.length()];
input.read(content);
Class c = unsafe.defineClass(null, content, 0, content.length,null,null);
c.getMethod("getId").invoke(c.newInstance(), null);
   

//分配一个8byte的内存
long address = unsafe.allocateMemory(8L);
//初始化内存填充1
unsafe.setMemory(address, 8L, (byte) 1);
//测试输出
System.out.println("add byte to memory:" + unsafe.getInt(address));
//设置0-3 4个byte为0x7fffffff
unsafe.putInt(address, 0x7fffffff);
//设置4-7 4个byte为0x80000000
unsafe.putInt(address + 4, 0x80000000);
//int占用4byte
System.out.println("add byte to memory:" + unsafe.getInt(address));
System.out.println("add byte to memory:" + unsafe.getInt(address + 4));

Data data = new Data();
data.setId(1L);
Field id = data.getClass().getDeclaredField("id");
long l = unsafe.objectFieldOffset(id);
id.setAccessible(true);
//比较并交换，比如id的值如果是所期望的值1，那么就替换为2，否则不做处理
unsafe.compareAndSwapLong(data,1L,1L,2L);
System.out.println(data.getId());

//get os address size
System.out.println("address size is :" + unsafe.addressSize());
//get os page size
System.out.println("page size is :" + unsafe.pageSize());
//int array base offset
System.out.println("unsafe array int base offset:" + Unsafe.ARRAY_INT_BASE_OFFSET);
   

Thread packThread = new Thread(() -> {
    long startTime = System.currentTimeMillis();
    //纳秒，相对时间park
    unsafe.park(false,3000000000L);
    //毫秒，绝对时间park
    //unsafe.park(true,System.currentTimeMillis()+3000); 
    System.out.println("main thread end,cost :"+(System.currentTimeMillis()-startTime)+"ms");
});
packThread.start();
TimeUnit.SECONDS.sleep(1);
//注释掉下一行后，线程3秒数后进行输出,否则在1秒后输出
unsafe.unpark(packThread);

public class SuperArray { 
    private static Unsafe unsafe = null;
    private static Field getUnsafe = null; 
    static {
        try {
            getUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
            getUnsafe.setAccessible(true);
            unsafe = (Unsafe) getUnsafe.get(null);
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    } 
 
    private final static int BYTE = 1; 
    private long size;
    private long address; 
    public SuperArray(long size) {
        this.size = size;
        address = unsafe.allocateMemory(size * BYTE);
    }
 
    public void set(long i, byte value) {
        unsafe.putByte(address + i * BYTE, value);
    }
 
    public int get(long idx) {
        return unsafe.getByte(address + idx * BYTE);
    }
 
    public long size() {
        return size;
    } 
 
    public static void main(String[] args) {
        long SUPER_SIZE = (long)Integer.MAX_VALUE * 2;
        SuperArray array = new SuperArray(SUPER_SIZE);
        System.out.println("Array size:" + array.size()); // 4294967294
        int sum=0;
        for (int i = 0; i < 100; i++) {
            array.set((long)Integer.MAX_VALUE + i, (byte)3);
            sum += array.get((long)Integer.MAX_VALUE + i);
        }
        System.out.println(sum);
    } 
}