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/** Flag that indicates whether this context is currently active */
private final AtomicBoolean active = new AtomicBoolean();
/** Flag that indicates whether this context has been closed already */
private final AtomicBoolean closed = new AtomicBoolean();
这个active 跟 closed代表着context状态，他是靠AtomicBoolean来进行操作的
//
// 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();
}
}
这个是个java的本地方法类，都是jdk底层对外提供操作内存的类，
如方法
public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);
这里可以举个例子来说明compareAndSet的作用，如支持并发的计数器，在进行计数的时候，首先读取当前的值，假设值为a，对当前值 + 1得到b，但是+1操作完以后，并不能直接修改原值为b，因为在进行+1操作的过程中，可能会有其它线程已经对原值进行了修改，所以在更新之前需要判断原 值是不是等于a，如果不等于a，说明有其它线程修改了，需要重新读取原值进行操作，如果等于a，说明在+1的操作过程中，没有其它线程来修改值，我们就可 以放心的更新原值了
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原文地址：http://my.oschina.net/u/867830/blog/407088
