concurrent atomic包 和 Unsafe 类

2017/12/17 技术分享

#concurrent atomic包 和 Unsafe 类

atomic 包

AtomicBoolean、AtomicInteger、AtomicIntegerArray(数组)、AtomicLong、AtomicLongArray、AtomicReference、AtomicReferenceArray类。 涉及到更新操作主要使用了 Unsafe 对 value 进行更新,value 是 volatile 修饰的;

public class AtomicInteger extends Number implements {
// setup to use Unsafe.compareAndSwapInt for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;

    static {
      try {
        valueOffset = unsafe.objectFieldOffset
      } catch (Exception ex) { throw new Error(ex); }

    private volatile int value;

Unsafe 类

并发编程下面有一个类是不能避免的,Unsafe,通过使用该类提供的一系列方法,可以执行更低层次不安全的操作;虽然这个类和它的所有方法都是 puplic 的,但是被限制使用再只有可信任的代码才能获得实例;注释大体是这个意思,应用层代码不提供实例化的方法,得通过以下方法调用

 private static Unsafe getUnsafeInstance() throws SecurityException,   NoSuchFieldException, IllegalArgumentException, IllegalAccessException {    
    Field theUnsafeInstance = Unsafe.class.getDeclaredField("theUnsafe");   
    return (Unsafe) theUnsafeInstance.get(Unsafe.class);

// offset 就是具体类的字段内存偏移量
static {    
    try {    
        unsafe = getUnsafeInstance();
        offset = unsafe.objectFieldOffset(UnsafeTest.class.getDeclaredField("flag"));
    } catch (NoSuchFieldException e) {        
    } catch (IllegalAccessException e) {

// 类似调用 Unsafe 的方法就可以了
private boolean doSwap(long offset, int expect, int update) {    
  return unsafe.compareAndSwapInt(this, offset, expect, update);

 * A collection of methods for performing low-level, unsafe operations.
 * Although the class and all methods are public, use of this class is
 * limited because only trusted code can obtain instances of it.
 * @author John R. Rose
 * @see #getUnsafe

public final class Unsafe {
	// 内部实例的名字,
    private static final Unsafe theUnsafe = new Unsafe();

    // 唯一的get获取实例方法,但是被限制了,根据类加载机制,一般的应用代码肯定会抛出异常,只能通过上面的方法获取
    public static Unsafe getUnsafe() {
        Class cc = Reflection.getCallerClass();
        if (cc.getClassLoader() != null)
            throw new SecurityException("Unsafe");
        return theUnsafe;


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