Unsafe例子
2024-08-27 22:36:13
Java和C++语言的一个重要区别就是Java中我们无法直接操作一块内存区域，不能像C++中那样可以自己申请内存和释放内存。Java中的Unsafe类为我们提供了类似C++手动管理内存的能力，不建议使用该类
（1）Unsafe对int，long ,Object的CAS操作
public class UnsafeTest {
public static void main(String[] args) throws Exception {
ExecutorService service = Executors.newFixedThreadPool();
Counter counter = new CASCounter();
long start = System.currentTimeMillis();
for (int i = ; i < ; i++) {
service.submit(new CounterRunnable(counter, ));
}
service.shutdown();
service.awaitTermination(, TimeUnit.HOURS);
System.out.println("counter : " + counter.getCount());
System.out.println("time elapse : " + (System.currentTimeMillis() - start));
}
public static Unsafe getUnsafe() throws IllegalAccessException {
Field unsafeField = Unsafe.class.getDeclaredFields()[];
unsafeField.setAccessible(true);
Unsafe unsafe = (Unsafe) unsafeField.get(null);
return unsafe;
}
interface Counter {
void increment();
long getCount();
}
static class StupidCounter implements Counter {
private long value = ;
@Override
public void increment() {
value++;
}
@Override
public long getCount() {
return value;
}
}
static class SynCounter implements Counter {
private long value = ;
@Override
public synchronized void increment() {
value++;
}
@Override
public long getCount() {
return value;
}
}
static class LockCounter implements Counter {
private long value = ;
private Lock lock = new ReentrantLock();
@Override
public void increment() {
lock.lock();
try {
value++;
} finally {
lock.unlock();
}
}
@Override
public long getCount() {
return value;
}
}
static class AtomicCounter implements Counter {
private AtomicLong value = new AtomicLong();
@Override
public void increment() {
value.incrementAndGet();
}
@Override
public long getCount() {
return value.get();
}
}
static class CASCounter implements Counter {
private Unsafe unsafe;
private long offset;
private volatile long value = ;
CASCounter() throws Exception {
unsafe = getUnsafe();
offset = unsafe.objectFieldOffset(CASCounter.class.getDeclaredField("value"));
}
@Override
public void increment() {
long current = value;
while (!unsafe.compareAndSwapLong(this, offset, value, value + )) {
current = value;
}
}
@Override
public long getCount() {
return value;
}
}
static class CounterRunnable implements Runnable {
Counter counter;
int num;
CounterRunnable(Counter counter, int num) {
this.counter = counter;
this.num = num;
}
@Override
public void run() {
for (int i = ; i < num; i++) {
counter.increment();
}
}
}
}
（2）可以绕过构造函数实例化对象
public class UnsafePlayer {
// 通过反射实例化Unsafe
public static Unsafe getUnsafe() throws IllegalAccessException {
Field unsafeField = Unsafe.class.getDeclaredFields()[];
unsafeField.setAccessible(true);
Unsafe unsafe = (Unsafe) unsafeField.get(null);
return unsafe;
}
public static void main(String[] args) throws Exception {
// 获取Unsafe
Unsafe unsafe = getUnsafe();
// 实例化Player
Player player = (Player) unsafe.allocateInstance(Player.class);
player.setName("li lei");
System.out.println(player.getName());
}
}
class Player {
private String name;
private Player() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
（3）直接修改内存的值
/**
*
* 直接修改了Foo对象的current值
*/
public class UnsafeTest2 { // 通过反射实例化Unsafe
public static Unsafe getUnsafe() throws IllegalAccessException {
Field unsafeField = Unsafe.class.getDeclaredFields()[];
unsafeField.setAccessible(true);
Unsafe unsafe = (Unsafe) unsafeField.get(null);
return unsafe;
} public static void main(String[] args) throws Exception {
Foo foo = new Foo();
Unsafe unsafe = getUnsafe();
Field field = Foo.class.getDeclaredField("current");
unsafe.putInt(foo, unsafe.objectFieldOffset(field), );
foo.work();
}
} class Foo { private int current = ; public void work() {
if (current == ) {
System.out.println(" I am working ");
}
} }
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