java不能直接访问操作系统底层，而是通过本地方法来访问。Unsafe类提供了硬件级别的原子操作，主要提供了以下功能：

1、通过Unsafe类可以分配内存，可以释放内存；

类中提供的3个本地方法allocateMemory、reallocateMemory、freeMemory分别用于分配内存，扩充内存和释放内存，与C语言中的3个方法对应。

2、可以定位对象某字段的内存位置，也可以修改对象的字段值，即使它是私有的；

public native long allocateMemory(long l);
public native long reallocateMemory(long l, long l1);
public native void freeMemory(long l);

字段的定位：

JAVA中对象的字段的定位可能通过staticFieldOffset方法实现，该方法返回给定field的内存地址偏移量，这个值对于给定的filed是唯一的且是固定不变的。

getIntVolatile方法获取对象中offset偏移地址对应的整型field的值,支持volatile load语义。

getLong方法获取对象中offset偏移地址对应的long型field的值

数组元素定位：

Unsafe类中有很多以BASE_OFFSET结尾的常量，比如ARRAY_INT_BASE_OFFSET，ARRAY_BYTE_BASE_OFFSET等，这些常量值是通过arrayBaseOffset方法得到的。arrayBaseOffset方法是一个本地方法，可以获取数组第一个元素的偏移地址。Unsafe类中还有很多以INDEX_SCALE结尾的常量，比如 ARRAY_INT_INDEX_SCALE ， ARRAY_BYTE_INDEX_SCALE等，这些常量值是通过arrayIndexScale方法得到的。arrayIndexScale方法也是一个本地方法，可以获取数组的转换因子，也就是数组中元素的增量地址。将arrayBaseOffset与arrayIndexScale配合使用，可以定位数组中每个元素在内存中的位置。

public final class Unsafe {
    public static final int ARRAY_INT_BASE_OFFSET;
    public static final int ARRAY_INT_INDEX_SCALE;

    public native long staticFieldOffset(Field field);
    public native int getIntVolatile(Object obj, long l);
    public native long getLong(Object obj, long l);
    public native int arrayBaseOffset(Class class1);
    public native int arrayIndexScale(Class class1);

    static
    {
        ARRAY_INT_BASE_OFFSET = theUnsafe.arrayBaseOffset([I);
        ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale([I);
    }
}

3、挂起与恢复

将一个线程进行挂起是通过park方法实现的，调用 park后，线程将一直阻塞直到超时或者中断等条件出现。unpark可以终止一个挂起的线程，使其恢复正常。整个并发框架中对线程的挂起操作被封装在 LockSupport类中，LockSupport类中有各种版本pack方法，但最终都调用了Unsafe.park()方法。

public class LockSupport {
    public static void unpark(Thread thread) {
        if (thread != null)
            unsafe.unpark(thread);
    }

    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        unsafe.park(false, 0L);
        setBlocker(t, null);
    }

    public static void parkNanos(Object blocker, long nanos) {
        if (nanos > 0) {
            Thread t = Thread.currentThread();
            setBlocker(t, blocker);
            unsafe.park(false, nanos);
            setBlocker(t, null);
        }
    }

    public static void parkUntil(Object blocker, long deadline) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        unsafe.park(true, deadline);
        setBlocker(t, null);
    }

    public static void park() {
        unsafe.park(false, 0L);
    }

    public static void parkNanos(long nanos) {
        if (nanos > 0)
            unsafe.park(false, nanos);
    }

    public static void parkUntil(long deadline) {
        unsafe.park(true, deadline);
    }
}

4、CAS操作

是通过compareAndSwapXXX方法实现的

/**
* 比较obj的offset处内存位置中的值和期望的值，如果相同则更新。此更新是不可中断的。
*
* @param obj 需要更新的对象
* @param offset obj中整型field的偏移量
* @param expect 希望field中存在的值
* @param update 如果期望值expect与field的当前值相同，设置filed的值为这个新值
* @return 如果field的值被更改返回true
*/
public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);

 

CAS操作有3个操作数，内存值M，预期值E，新值U，如果M==E，则将内存值修改为B，否则啥都不做。

 

另外附上Unsafe 源代码：

//// Source code recreated from a .class file by IntelliJ IDEA// (powered by Fernflower decompiler)//package sun.misc;import java.lang.reflect.Field;import java.lang.reflect.Modifier;import java.security.ProtectionDomain;import sun.reflect.CallerSensitive;import sun.reflect.Reflection;public final class Unsafe {    private static final Unsafe theUnsafe;    public static final int INVALID_FIELD_OFFSET = -1;    public static final int ARRAY_BOOLEAN_BASE_OFFSET;    public static final int ARRAY_BYTE_BASE_OFFSET;    public static final int ARRAY_SHORT_BASE_OFFSET;    public static final int ARRAY_CHAR_BASE_OFFSET;    public static final int ARRAY_INT_BASE_OFFSET;    public static final int ARRAY_LONG_BASE_OFFSET;    public static final int ARRAY_FLOAT_BASE_OFFSET;    public static final int ARRAY_DOUBLE_BASE_OFFSET;    public static final int ARRAY_OBJECT_BASE_OFFSET;    public static final int ARRAY_BOOLEAN_INDEX_SCALE;    public static final int ARRAY_BYTE_INDEX_SCALE;    public static final int ARRAY_SHORT_INDEX_SCALE;    public static final int ARRAY_CHAR_INDEX_SCALE;    public static final int ARRAY_INT_INDEX_SCALE;    public static final int ARRAY_LONG_INDEX_SCALE;    public static final int ARRAY_FLOAT_INDEX_SCALE;    public static final int ARRAY_DOUBLE_INDEX_SCALE;    public static final int ARRAY_OBJECT_INDEX_SCALE;    public static final int ADDRESS_SIZE;    private static native void registerNatives();    private Unsafe() {    }    @CallerSensitive    public static Unsafe getUnsafe() {        Class var0 = Reflection.getCallerClass();        if(var0.getClassLoader() != null) {            throw new SecurityException("Unsafe");        } else {            return theUnsafe;        }    }    public native int getInt(Object var1, long var2);    public native void putInt(Object var1, long var2, int var4);    public native Object getObject(Object var1, long var2);    public native void putObject(Object var1, long var2, Object var4);    public native boolean getBoolean(Object var1, long var2);    public native void putBoolean(Object var1, long var2, boolean var4);    public native byte getByte(Object var1, long var2);    public native void putByte(Object var1, long var2, byte var4);    public native short getShort(Object var1, long var2);    public native void putShort(Object var1, long var2, short var4);    public native char getChar(Object var1, long var2);    public native void putChar(Object var1, long var2, char var4);    public native long getLong(Object var1, long var2);    public native void putLong(Object var1, long var2, long var4);    public native float getFloat(Object var1, long var2);    public native void putFloat(Object var1, long var2, float var4);    public native double getDouble(Object var1, long var2);    public native void putDouble(Object var1, long var2, double var4);    /** @deprecated */    @Deprecated    public int getInt(Object var1, int var2) {        return this.getInt(var1, (long)var2);    }    /** @deprecated */    @Deprecated    public void putInt(Object var1, int var2, int var3) {        this.putInt(var1, (long)var2, var3);    }    /** @deprecated */    @Deprecated    public Object getObject(Object var1, int var2) {        return this.getObject(var1, (long)var2);    }    /** @deprecated */    @Deprecated    public void putObject(Object var1, int var2, Object var3) {        this.putObject(var1, (long)var2, var3);    }    /** @deprecated */    @Deprecated    public boolean getBoolean(Object var1, int var2) {        return this.getBoolean(var1, (long)var2);    }    /** @deprecated */    @Deprecated    public void putBoolean(Object var1, int var2, boolean var3) {        this.putBoolean(var1, (long)var2, var3);    }    /** @deprecated */    @Deprecated    public byte getByte(Object var1, int var2) {        return this.getByte(var1, (long)var2);    }    /** @deprecated */    @Deprecated    public void putByte(Object var1, int var2, byte var3) {        this.putByte(var1, (long)var2, var3);    }    /** @deprecated */    @Deprecated    public short getShort(Object var1, int var2) {        return this.getShort(var1, (long)var2);    }    /** @deprecated */    @Deprecated    public void putShort(Object var1, int var2, short var3) {        this.putShort(var1, (long)var2, var3);    }    /** @deprecated */    @Deprecated    public char getChar(Object var1, int var2) {        return this.getChar(var1, (long)var2);    }    /** @deprecated */    @Deprecated    public void putChar(Object var1, int var2, char var3) {        this.putChar(var1, (long)var2, var3);    }    /** @deprecated */    @Deprecated    public long getLong(Object var1, int var2) {        return this.getLong(var1, (long)var2);    }    /** @deprecated */    @Deprecated    public void putLong(Object var1, int var2, long var3) {        this.putLong(var1, (long)var2, var3);    }    /** @deprecated */    @Deprecated    public float getFloat(Object var1, int var2) {        return this.getFloat(var1, (long)var2);    }    /** @deprecated */    @Deprecated    public void putFloat(Object var1, int var2, float var3) {        this.putFloat(var1, (long)var2, var3);    }    /** @deprecated */    @Deprecated    public double getDouble(Object var1, int var2) {        return this.getDouble(var1, (long)var2);    }    /** @deprecated */    @Deprecated    public void putDouble(Object var1, int var2, double var3) {        this.putDouble(var1, (long)var2, var3);    }    public native byte getByte(long var1);    public native void putByte(long var1, byte var3);    public native short getShort(long var1);    public native void putShort(long var1, short var3);    public native char getChar(long var1);    public native void putChar(long var1, char var3);    public native int getInt(long var1);    public native void putInt(long var1, int var3);    public native long getLong(long var1);    public native void putLong(long var1, long var3);    public native float getFloat(long var1);    public native void putFloat(long var1, float var3);    public native double getDouble(long var1);    public native void putDouble(long var1, double var3);    public native long getAddress(long var1);    public native void putAddress(long var1, long var3);    public native long allocateMemory(long var1);    public native long reallocateMemory(long var1, long var3);    public native void setMemory(Object var1, long var2, long var4, byte var6);    public void setMemory(long var1, long var3, byte var5) {        this.setMemory((Object)null, var1, var3, var5);    }    public native void copyMemory(Object var1, long var2, Object var4, long var5, long var7);    public void copyMemory(long var1, long var3, long var5) {        this.copyMemory((Object)null, var1, (Object)null, var3, var5);    }    public native void freeMemory(long var1);    /** @deprecated */    @Deprecated    public int fieldOffset(Field var1) {        return Modifier.isStatic(var1.getModifiers())?(int)this.staticFieldOffset(var1):(int)this.objectFieldOffset(var1);    }    /** @deprecated */    @Deprecated    public Object staticFieldBase(Class var1) {        Field[] var2 = var1.getDeclaredFields();        for(int var3 = 0; var3 < var2.length; ++var3) {            if(Modifier.isStatic(var2[var3].getModifiers())) {                return this.staticFieldBase(var2[var3]);            }        }        return null;    }    public native long staticFieldOffset(Field var1);    public native long objectFieldOffset(Field var1);    public native Object staticFieldBase(Field var1);    public native boolean shouldBeInitialized(Class
       var1);    public native void ensureClassInitialized(Class var1);    public native int arrayBaseOffset(Class var1);    public native int arrayIndexScale(Class var1);    public native int addressSize();    public native int pageSize();    public native Class defineClass(String var1, byte[] var2, int var3, int var4, ClassLoader var5, ProtectionDomain var6);    /** @deprecated */    @Deprecated    @CallerSensitive    public native Class defineClass(String var1, byte[] var2, int var3, int var4);    public native Class defineAnonymousClass(Class var1, byte[] var2, Object[] var3);    public native Object allocateInstance(Class var1) throws InstantiationException;    public native void monitorEnter(Object var1);    public native void monitorExit(Object var1);    public native boolean tryMonitorEnter(Object var1);    public native void throwException(Throwable var1);    public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);    public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);    public final native boolean compareAndSwapLong(Object var1, long var2, long var4, long var6);    public native Object getObjectVolatile(Object var1, long var2);    public native void putObjectVolatile(Object var1, long var2, Object var4);    public native int getIntVolatile(Object var1, long var2);    public native void putIntVolatile(Object var1, long var2, int var4);    public native boolean getBooleanVolatile(Object var1, long var2);    public native void putBooleanVolatile(Object var1, long var2, boolean var4);    public native byte getByteVolatile(Object var1, long var2);    public native void putByteVolatile(Object var1, long var2, byte var4);    public native short getShortVolatile(Object var1, long var2);    public native void putShortVolatile(Object var1, long var2, short var4);    public native char getCharVolatile(Object var1, long var2);    public native void putCharVolatile(Object var1, long var2, char var4);    public native long getLongVolatile(Object var1, long var2);    public native void putLongVolatile(Object var1, long var2, long var4);    public native float getFloatVolatile(Object var1, long var2);    public native void putFloatVolatile(Object var1, long var2, float var4);    public native double getDoubleVolatile(Object var1, long var2);    public native void putDoubleVolatile(Object var1, long var2, double var4);    public native void putOrderedObject(Object var1, long var2, Object var4);    public native void putOrderedInt(Object var1, long var2, int var4);    public native void putOrderedLong(Object var1, long var2, long var4);    public native void unpark(Object var1);    public native void park(boolean var1, long var2);    public native int getLoadAverage(double[] var1, int var2);    static {        registerNatives();        Reflection.registerMethodsToFilter(Unsafe.class, new String[]{"getUnsafe"});        theUnsafe = new Unsafe();        ARRAY_BOOLEAN_BASE_OFFSET = theUnsafe.arrayBaseOffset(boolean[].class);        ARRAY_BYTE_BASE_OFFSET = theUnsafe.arrayBaseOffset(byte[].class);        ARRAY_SHORT_BASE_OFFSET = theUnsafe.arrayBaseOffset(short[].class);        ARRAY_CHAR_BASE_OFFSET = theUnsafe.arrayBaseOffset(char[].class);        ARRAY_INT_BASE_OFFSET = theUnsafe.arrayBaseOffset(int[].class);        ARRAY_LONG_BASE_OFFSET = theUnsafe.arrayBaseOffset(long[].class);        ARRAY_FLOAT_BASE_OFFSET = theUnsafe.arrayBaseOffset(float[].class);        ARRAY_DOUBLE_BASE_OFFSET = theUnsafe.arrayBaseOffset(double[].class);        ARRAY_OBJECT_BASE_OFFSET = theUnsafe.arrayBaseOffset(Object[].class);        ARRAY_BOOLEAN_INDEX_SCALE = theUnsafe.arrayIndexScale(boolean[].class);        ARRAY_BYTE_INDEX_SCALE = theUnsafe.arrayIndexScale(byte[].class);        ARRAY_SHORT_INDEX_SCALE = theUnsafe.arrayIndexScale(short[].class);        ARRAY_CHAR_INDEX_SCALE = theUnsafe.arrayIndexScale(char[].class);        ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale(int[].class);        ARRAY_LONG_INDEX_SCALE = theUnsafe.arrayIndexScale(long[].class);        ARRAY_FLOAT_INDEX_SCALE = theUnsafe.arrayIndexScale(float[].class);        ARRAY_DOUBLE_INDEX_SCALE = theUnsafe.arrayIndexScale(double[].class);        ARRAY_OBJECT_INDEX_SCALE = theUnsafe.arrayIndexScale(Object[].class);        ADDRESS_SIZE = theUnsafe.addressSize();    }}