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JAVA Unsafe 类介绍

news 来源：原创 2024/9/19 13:04:37

Unsafe 类提供了低级别、不安全的原始操作，这些操作可以直接访问内存。由于这些操作非常强大，可以直接操作内存，因此也非常危险。如果使用不当，可能会导致严重的内存泄露问题。

Unsafe 类的主要功能包括：

直接内存访问：可以分配和释放内存，创建和操作对象。

  private static final int BUFFER_SIZE = 1024;// 获取Unsafe实例private static final Unsafe unsafe = getUnsafeInstance();// 分配内存private static final long buffer = unsafe.allocateMemory(BUFFER_SIZE);private static Unsafe getUnsafeInstance() {try {Field field = Unsafe.class.getDeclaredField("theUnsafe");field.setAccessible(true);return (Unsafe) field.get(null);} catch (Exception e) {e.printStackTrace();}return null;}public static void main(String[] args) {// 使用直接内存unsafe.setMemory(buffer, BUFFER_SIZE, (byte) 0);// ... 进行其他操作// 释放内存unsafe.freeMemory(buffer);}

线程和锁：提供了线程的挂起和恢复、定时等待等操作。

    private static final Unsafe UNSAFE;private static final long THREAD_PTR;static {try {Field field = Unsafe.class.getDeclaredField("theUnsafe");field.setAccessible(true);UNSAFE = (Unsafe) field.get(null);Thread currentThread = Thread.currentThread();THREAD_PTR = UNSAFE.getLong(currentThread, UNSAFE.objectFieldOffset(Thread.class.getDeclaredField("threadStatus")));} catch (Exception e) {throw new Error(e);}}public static void parkThread() {UNSAFE.park(false, 0L);}public static void unparkThread() {UNSAFE.unpark(Thread.currentThread());}public static void main(String[] args) throws InterruptedException {Thread thread = new Thread(() -> {System.out.println("Thread is going to be parked");parkThread();System.out.println("Thread is unparked");});thread.start();Thread.sleep(1000); // Wait for the thread to start and print the first messageunparkThread(); // Unpark the thread}

可见性操作：提供了一些方法来保证线程间的可见性。

  private static final Unsafe UNSAFE;private static final long VALUE_OFFSET;static {try {Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");theUnsafe.setAccessible(true);UNSAFE = (Unsafe) theUnsafe.get(null);VALUE_OFFSET = UNSAFE.objectFieldOffset(UnsafeVisibilityExample.class.getDeclaredField("value"));} catch (Exception e) {throw new Error(e);}}private volatile int value = 0;public void increaseValue() {UNSAFE.putOrderedInt(this, VALUE_OFFSET, value + 1);}public int getValue() {return value;}public static void main(String[] args) throws InterruptedException {UnsafeVisibilityExample example = new UnsafeVisibilityExample();Thread t1 = new Thread(() -> {for (int i = 0; i < 10000; i++) {example.increaseValue();}});Thread t2 = new Thread(() -> {while (example.getValue() < 10000) {// Busy wait until condition is met}});t1.start();t2.start();t1.join();t2.join();System.out.println("Value after both threads have finished: " + example.getValue());}

CAS 操作：提供了 CompareAndSwap 等原子操作。

    private static final Unsafe UNSAFE;private static final long VALUE_OFFSET;static {try {Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");theUnsafe.setAccessible(true);UNSAFE = (Unsafe) theUnsafe.get(null);VALUE_OFFSET = UNSAFE.objectFieldOffset(UnsafeTest.class.getDeclaredField("value"));} catch (Exception e) {throw new Error(e);}}private volatile int value;public static void main(String[] args) {UnsafeTest unsafeTest = new UnsafeTest();unsafeTest.value = 10;System.out.println("Before: " + unsafeTest.value);boolean success = UNSAFE.compareAndSwapInt(unsafeTest, VALUE_OFFSET, 10, 20);System.out.println("Success: " + success);System.out.println("After: " + unsafeTest.value);}