Unsafe类

Java魔法类：Unsafe应用解析

Unsafe是位于sun.misc包下的一个类，主要提供一些用于执行低级别、不安全操作的方法，如直接访问系统内存资源、自主管理内存资源等，这些方法在提升Java运行效率、增强Java语言底层资源操作能力方面起到了很大的作用。但由于Unsafe类使Java语言拥有了类似C语言指针一样操作内存空间的能力，这无疑也增加了程序发生相关指针问题的风险。在程序中过度、不正确使用Unsafe类会使得程序出错的概率变大，使得Java这种安全的语言变得不再"安全"，因此对Unsafe的使用一定要慎重。注：本文对sun.misc.Unsafe公共API功能及相关应用场景进行介绍。如下Unsafe源码所示，Unsafe类为一单例实现，提供静态方法getUnsafe获取Unsafe实例，当且仅当调用getUnsafe方法的类为引导类加载器所加载时才合法，否则抛出SecurityException异常。 public final class Unsafe { // 单例对象 private static final Unsafe theUnsafe; private Unsafe() { } @CallerSensitive public static Unsafe getUnsafe() { Class var0 = Reflection.getCallerClass(); // 仅在引导类加载器`BootstrapClassLoader`加载时才合法 if(!VM.isSystemDomainLoader(var0.getClassLoader())) { throw new SecurityException("Unsafe"); } else { return theUnsafe; } } } 那如若想使用这个类，该如何获取其实例？有如下两个可行方案。其一，从 getUnsafe方法的使用限制条件出发，通过Java命令行命令-Xbootclasspath/a把调用Unsafe相关方法的类A所在jar包路径追加到默认的bootstrap路径中，使得A被引导类加载器加载，从而通过 Unsafe.getUnsafe 方法安全的获取Unsafe实例。 java

https://tech.meituan.com/2019/02/14/talk-about-java-magic-class-unsafe.html

Unsafe对象

Unsafe 对象提供了非常底层的，操作内存、线程的方法，Unsafe 对象不能直接调用，只能通过反射获得


public class UnsafeAccessor {
    static Unsafe unsafe;

    static {
        try {
            Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
            theUnsafe.setAccessible(true);
            unsafe = (Unsafe) theUnsafe.get(null);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            throw new Error(e);
        }
    }

    static Unsafe getUnsafe() {
        return unsafe;
    }
}


public class TestUnsafe {

    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
        theUnsafe.setAccessible(true);
        Unsafe unsafe = (Unsafe) theUnsafe.get(null);

        System.out.println(unsafe);

        // 1. 获取域的偏移地址
        long idOffset = unsafe.objectFieldOffset(Teacher.class.getDeclaredField("id"));
        long nameOffset = unsafe.objectFieldOffset(Teacher.class.getDeclaredField("name"));

        Teacher t = new Teacher();
        // 2. 执行 cas 操作
        unsafe.compareAndSwapInt(t, idOffset, 0, 1);
        unsafe.compareAndSwapObject(t, nameOffset, null, "张三");

        // 3. 验证
        System.out.println(t);
    }
}
@Data
class Teacher {
    volatile int id;
    volatile String name;
}

Park & Unpark

基本使用

它们是 LockSupport 类中的方法


// 暂停当前线程 
LockSupport.park();
// 恢复某个线程的运行 
LockSupport.unpark(暂停线程对象)

先park 后 unpark


@Slf4j(topic = "c.TestParkUnpark")
public class TestParkUnpark {
    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            log.debug("start...");
            sleep(1);
            log.debug("park...");
            LockSupport.park();
            log.debug("resume...");
        }, "t1");
        t1.start();

        sleep(2);
        log.debug("unpark...");
        LockSupport.unpark(t1);
    }
}
// 输出
// 13:34:19.631 c.TestParkUnpark [t1] - start...
// 13:34:20.642 c.TestParkUnpark [t1] - park...
// 13:34:21.635 c.TestParkUnpark [main] - unpark...
// 13:34:21.635 c.TestParkUnpark [t1] - resume...

先unpark 后park

park失效


@Slf4j(topic = "c.TestParkUnpark")
public class TestParkUnpark {
    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            log.debug("start...");
            sleep(2);
            log.debug("park...");
            LockSupport.park();
            log.debug("resume...");
        }, "t1");
        t1.start();

        sleep(1);
        log.debug("unpark...");
        LockSupport.unpark(t1);
    }
}
// 输出
//13:35:28.872 c.TestParkUnpark [t1] - start...
//13:35:29.875 c.TestParkUnpark [main] - unpark...
//13:35:30.883 c.TestParkUnpark [t1] - park...
//13:35:30.883 c.TestParkUnpark [t1] - resume...