jdk-Atomic源码学习
源码下载：https://gitee.com/hong99/jdk8
Atomic了解
atomic是并发框架中的一员，所属位置：java.util.concurrent.atomic 该类主要是用来解决内存可见性、有序、线程安全，当然底层也是通过cas来实现，所以性能相同步锁也是高不少。
Atomic解决了什么问题?
并发的三大特性：原子性、可见性、有序性
没错atomic就是解决以上三种特性而专门设计的一种在高并发下线程安全实现。主要的操作有三步，
首首：获取锁，这里是通过cas进行实现，并且是带版本的方式，解决了aba的问题；
其次：将所获取的值、版本号、新值进行更新，若成则进返回（同存可见通知其它处理器），若失败则直接返回结果;
最后：将结果返回给操作线程或让出线程，或中间让出线程。
Atomic相关基础学习
atomic相关类型如下：
基本类型：
AtomicLong | 长整型原子 |
|---|---|
AtomicInteger | 整型原子 |
AtomicBoolean | 布尔类型 |
引用类型：
AtomicReference | 引用类型（存在aba） |
|---|---|
AtomicStampedReference | 更新带版本号的引用类型 |
AtomicMarkableReference | 更新带标主的引用类型 |
数组类型：
AtomicLongArray | 更新长整型数组里的元素 |
|---|---|
AtomicIntegerArray | 更新整型数组里的元素 |
AtomicReferenceArray | 更新引用类型里面的元素 |
字段类型：
AtomicIntegerFieldUpdater | 更新整型字段 |
|---|---|
AtomicLongFieldUpdater | 更新长整型字段 |
AtomicReferenceFieldUpdater | 更新引用类型字段 |
类加器类型：
DoubleAccumulator | double累加器 |
|---|---|
DoubleAdder | double累加器 |
LongAccumulator | long类型累加器 |
LongAdder | long累加器 |
基础使用：
AtomicInteger atomicInteger = new AtomicInteger();
for (int i = 0; i < 10; i++) {
new Thread(new Runnable() {
@Override
public void run() {
System.out.println(atomicInteger.getAndIncrement());
}
}).start();
}结果
1
3
2
4
0
5
6
7
8
9可以看出所有都是独立的，而不是重复。所以是线程安全跟内存可见的。
AtomicBoolean atomicBoolean = new AtomicBoolean();
while (!atomicBoolean.get()){
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(atomicBoolean.getAndSet(true));
}
}).start();
}
System.out.println("结束了！");结果
false
结束了！
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true引用类型学习
AtomicReference<User> atomicReference = new AtomicReference<User>();
User user = new User("1","用户1");
User user2 = new User("2","用户2");
atomicReference.set(user);
boolean b = atomicReference.compareAndSet(user, user2);
System.out.println(b);
System.out.println(atomicReference.get().toString());结果
true
User{userName='2', userId='用户2'}其他的大至同上，所以不一一了解，需要了解的可以自行了解。
Atomic源码学习
java.util.concurrent.atomic.AtomicInteger 源码学习
//整型原子类实现
public class AtomicInteger extends Number implements java.io.Serializable {
private static final long serialVersionUID = 6214790243416807050L;
// 设置使用Unsafe.compareAndSwapInt进行更新
private static final Unsafe unsafe = Unsafe.getUnsafe();
//存放变量value值的偏移量值
private static final long valueOffset;
//初始值
static {
try {
//初始化偏移量
valueOffset = unsafe.objectFieldOffset
(AtomicInteger.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}
private volatile int value;
//带参数的构造方法
public AtomicInteger(int initialValue) {
value = initialValue;
}
//空构造方法
public AtomicInteger() {
}
//获取当前值
public final int get() {
return value;
}
//设置新值
public final void set(int newValue) {
value = newValue;
}
//设置值（不带屏障会导致数据一致性问题）
public final void lazySet(int newValue) {
unsafe.putOrderedInt(this, valueOffset, newValue);
}
//原子设置新值并返回旧值
public final int getAndSet(int newValue) {
return unsafe.getAndSetInt(this, valueOffset, newValue);
}
//对比值并更新返回 true为成功 false为失败
public final boolean compareAndSet(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}
//如果当前值等于期望则进行更新并返回 true 否则返回false
public final boolean weakCompareAndSet(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}
//自增并返回旧值
public final int getAndIncrement() {
return unsafe.getAndAddInt(this, valueOffset, 1);
}
//自减并返回旧值
public final int getAndDecrement() {
return unsafe.getAndAddInt(this, valueOffset, -1);
}
//原子的添加指定的值并返回旧值
public final int getAndAdd(int delta) {
return unsafe.getAndAddInt(this, valueOffset, delta);
}
//自增1并返回新值
public final int incrementAndGet() {
return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
}
//自减1并返回新值
public final int decrementAndGet() {
return unsafe.getAndAddInt(this, valueOffset, -1) - 1;
}
//新增指定的值并返回新值
public final int addAndGet(int delta) {
return unsafe.getAndAddInt(this, valueOffset, delta) + delta;
}
//通过函数方式进行更新值并返回旧值
public final int getAndUpdate(IntUnaryOperator updateFunction) {
int prev, next;
do {
prev = get();
next = updateFunction.applyAsInt(prev);
} while (!compareAndSet(prev, next));
return prev;
}
//同上类型，返回新值
public final int updateAndGet(IntUnaryOperator updateFunction) {
int prev, next;
do {
prev = get();
next = updateFunction.applyAsInt(prev);
} while (!compareAndSet(prev, next));
return next;
}
//x为新值及函数进行更新返回之前的值
public final int getAndAccumulate(int x,
IntBinaryOperator accumulatorFunction) {
int prev, next;
do {
prev = get();
next = accumulatorFunction.applyAsInt(prev, x);
} while (!compareAndSet(prev, next));
return prev;
}
//同上类似，返回更新的新值
public final int accumulateAndGet(int x,
IntBinaryOperator accumulatorFunction) {
int prev, next;
do {
prev = get();
next = accumulatorFunction.applyAsInt(prev, x);
} while (!compareAndSet(prev, next));
return next;
}
public String toString() {
return Integer.toString(get());
}
//获取当前值
public int intValue() {
return get();
}
//获取长整型的当前值
public long longValue() {
return (long)get();
}
//获取浮点数的当前值
public float floatValue() {
return (float)get();
}
//获取double类型当前值
public double doubleValue() {
return (double)get();
}
}可以看到这个AtomicInteger非常简单，主要是利用Unsafe进行实现，不过Unsafe大部分都是c++代码，这里Java层面看不了，需要去深入c++，这里不再细究。
当然AtomicBoolean与AtomicLong都差不多这里不深入。
java.util.concurrent.atomic.AtomicReference 源码学习
package java.util.concurrent.atomic;
import java.util.function.UnaryOperator;
import java.util.function.BinaryOperator;
import sun.misc.Unsafe;
//原子引用（存在aba问题）
public class AtomicReference<V> implements java.io.Serializable {
private static final long serialVersionUID = -1848883965231344442L;
//使用Unsafe做为更新的实现
private static final Unsafe unsafe = Unsafe.getUnsafe();
//存放变量value值的偏移量值
private static final long valueOffset;
//初始化
static {
try {
//初始化偏移量
valueOffset = unsafe.objectFieldOffset
(AtomicReference.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}
//可见的值
private volatile V value;
//构造方法，带初始参
public AtomicReference(V initialValue) {
value = initialValue;
}
//构造方法
public AtomicReference() {
}
//获取当前值
public final V get() {
return value;
}
//设置新值方法
public final void set(V newValue) {
value = newValue;
}
//延迟的值设置方法
public final void lazySet(V newValue) {
unsafe.putOrderedObject(this, valueOffset, newValue);
}
//当前值等于期望则更新成功，否则失败
public final boolean compareAndSet(V expect, V update) {
return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
}
//不保证排序，当前值等于期望则更新成功，否则失败
public final boolean weakCompareAndSet(V expect, V update) {
return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
}
//原子的设置值，并返回旧值
@SuppressWarnings("unchecked")
public final V getAndSet(V newValue) {
return (V)unsafe.getAndSetObject(this, valueOffset, newValue);
}
//通过函数进行更新，并返回旧值
public final V getAndUpdate(UnaryOperator<V> updateFunction) {
V prev, next;
do {
prev = get();
next = updateFunction.apply(prev);
} while (!compareAndSet(prev, next));
return prev;
}
//同上类似，返回新值
public final V updateAndGet(UnaryOperator<V> updateFunction) {
V prev, next;
do {
prev = get();
next = updateFunction.apply(prev);
} while (!compareAndSet(prev, next));
return next;
}
//通过函数更新新值并返回旧值
public final V getAndAccumulate(V x,
BinaryOperator<V> accumulatorFunction) {
V prev, next;
do {
prev = get();
next = accumulatorFunction.apply(prev, x);
} while (!compareAndSet(prev, next));
return prev;
}
//同上类似，返回新值
public final V accumulateAndGet(V x,
BinaryOperator<V> accumulatorFunction) {
V prev, next;
do {
prev = get();
next = accumulatorFunction.apply(prev, x);
} while (!compareAndSet(prev, next));
return next;
}
/**
* Returns the String representation of the current value.
* @return the String representation of the current value
*/
public String toString() {
return String.valueOf(get());
}
}注意当前这个引用类型的是存在aba的问题，如果需要使用尽量还是用AtomicStampedReference详细，请看看这个的源码实现，也挺简单的。
java.util.concurrent.atomic.AtomicIntegerFieldUpdater 源码学习
package java.util.concurrent.atomic;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.security.AccessController;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.Objects;
import java.util.function.IntBinaryOperator;
import java.util.function.IntUnaryOperator;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
//对象整型类型的
public abstract class AtomicIntegerFieldUpdater<T> {
//返回一个初始化对象
@CallerSensitive
public static <U> AtomicIntegerFieldUpdater<U> newUpdater(Class<U> tclass,
String fieldName) {
return new AtomicIntegerFieldUpdaterImpl<U>
(tclass, fieldName, Reflection.getCallerClass());
}
//无参构造方法
protected AtomicIntegerFieldUpdater() {
}
//当前值等于期望值进行更新并返回true
public abstract boolean compareAndSet(T obj, int expect, int update);
//不保证排序 当前值等于期望值进行更新
public abstract boolean weakCompareAndSet(T obj, int expect, int update);
//设置新值
public abstract void set(T obj, int newValue);
//延迟设置新值
public abstract void lazySet(T obj, int newValue);
//获取当前值方法
public abstract int get(T obj);
//设置新值并返回旧值
public int getAndSet(T obj, int newValue) {
int prev;
do {
prev = get(obj);
} while (!compareAndSet(obj, prev, newValue));
return prev;
}
//自增1并返回旧值
public int getAndIncrement(T obj) {
int prev, next;
do {
prev = get(obj);
next = prev + 1;
} while (!compareAndSet(obj, prev, next));
return prev;
}
//自减1并返回旧值
public int getAndDecrement(T obj) {
int prev, next;
do {
prev = get(obj);
next = prev - 1;
} while (!compareAndSet(obj, prev, next));
return prev;
}
//原子的将delta的值加上并返回旧值
public int getAndAdd(T obj, int delta) {
int prev, next;
do {
prev = get(obj);
next = prev + delta;
} while (!compareAndSet(obj, prev, next));
return prev;
}
//自增1并返回新值
public int incrementAndGet(T obj) {
int prev, next;
do {
prev = get(obj);
next = prev + 1;
} while (!compareAndSet(obj, prev, next));
return next;
}
//自减1并返回新值
public int decrementAndGet(T obj) {
int prev, next;
do {
prev = get(obj);
next = prev - 1;
} while (!compareAndSet(obj, prev, next));
return next;
}
//新增当前值加上delta并返回新值
public int addAndGet(T obj, int delta) {
int prev, next;
do {
prev = get(obj);
next = prev + delta;
} while (!compareAndSet(obj, prev, next));
return next;
}
//更新并返回旧值
public final int getAndUpdate(T obj, IntUnaryOperator updateFunction) {
int prev, next;
do {
prev = get(obj);
next = updateFunction.applyAsInt(prev);
} while (!compareAndSet(obj, prev, next));
return prev;
}
//更新并返回新值
public final int updateAndGet(T obj, IntUnaryOperator updateFunction) {
int prev, next;
do {
prev = get(obj);
next = updateFunction.applyAsInt(prev);
} while (!compareAndSet(obj, prev, next));
return next;
}
//通过函数的方法进行更新并返回旧值
public final int getAndAccumulate(T obj, int x,
IntBinaryOperator accumulatorFunction) {
int prev, next;
do {
prev = get(obj);
next = accumulatorFunction.applyAsInt(prev, x);
} while (!compareAndSet(obj, prev, next));
return prev;
}
//同上类似返回新值
public final int accumulateAndGet(T obj, int x,
IntBinaryOperator accumulatorFunction) {
int prev, next;
do {
prev = get(obj);
next = accumulatorFunction.applyAsInt(prev, x);
} while (!compareAndSet(obj, prev, next));
return next;
}
//具体实现的方法
private static final class AtomicIntegerFieldUpdaterImpl<T>
extends AtomicIntegerFieldUpdater<T> {
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
private final long offset;
//
private final Class<?> cclass;
/** class holding the field */
private final Class<T> tclass;
//构造实现方法
AtomicIntegerFieldUpdaterImpl(final Class<T> tclass,
final String fieldName,
final Class<?> caller) {
final Field field;
final int modifiers;
try {
//反射获取到的属值信息
field = AccessController.doPrivileged(
new PrivilegedExceptionAction<Field>() {
public Field run() throws NoSuchFieldException {
return tclass.getDeclaredField(fieldName);
}
});
//用于判断是不是public
modifiers = field.getModifiers();
sun.reflect.misc.ReflectUtil.ensureMemberAccess(
caller, tclass, null, modifiers);
ClassLoader cl = tclass.getClassLoader();
ClassLoader ccl = caller.getClassLoader();
if ((ccl != null) && (ccl != cl) &&
((cl == null) || !isAncestor(cl, ccl))) {
sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
}
} catch (PrivilegedActionException pae) {
throw new RuntimeException(pae.getException());
} catch (Exception ex) {
throw new RuntimeException(ex);
}
//类型判断是不是int
if (field.getType() != int.class)
throw new IllegalArgumentException("Must be integer type");
//必须被volatile修饰
if (!Modifier.isVolatile(modifiers))
throw new IllegalArgumentException("Must be volatile type");
//判断获取caller或tclass 通过判断是否在同包或private或者为超类
this.cclass = (Modifier.isProtected(modifiers) &&
tclass.isAssignableFrom(caller) &&
!isSamePackage(tclass, caller))
? caller : tclass;
//目标类
this.tclass = tclass;
//偏移量
this.offset = U.objectFieldOffset(field);
}
//判断第二个类加载器被第一个已加载，如果是返回true（多亲委派）
private static boolean isAncestor(ClassLoader first, ClassLoader second) {
ClassLoader acl = first;
do {
acl = acl.getParent();
if (second == acl) {
return true;
}
} while (acl != null);
return false;
}
//两个是否在同一个包和修饰符一样
private static boolean isSamePackage(Class<?> class1, Class<?> class2) {
return class1.getClassLoader() == class2.getClassLoader()
&& Objects.equals(getPackageName(class1), getPackageName(class2));
}
//获取包名称
private static String getPackageName(Class<?> cls) {
String cn = cls.getName();
int dot = cn.lastIndexOf('.');
return (dot != -1) ? cn.substring(0, dot) : "";
}
//判断是cclass的实例，如果不是抛出异常
private final void accessCheck(T obj) {
if (!cclass.isInstance(obj))
throwAccessCheckException(obj);
}
//若受保护异常则跑出异常
private final void throwAccessCheckException(T obj) {
if (cclass == tclass)
throw new ClassCastException();
else
throw new RuntimeException(
new IllegalAccessException(
"Class " +
cclass.getName() +
" can not access a protected member of class " +
tclass.getName() +
" using an instance of " +
obj.getClass().getName()));
}
//对比值并更新返回 true为成功 false为失败
public final boolean compareAndSet(T obj, int expect, int update) {
accessCheck(obj);
return U.compareAndSwapInt(obj, offset, expect, update);
}
//不带排序 如果当前值等于期望则进行更新并返回 true 否则返回false
public final boolean weakCompareAndSet(T obj, int expect, int update) {
accessCheck(obj);
return U.compareAndSwapInt(obj, offset, expect, update);
}
//设置新值
public final void set(T obj, int newValue) {
accessCheck(obj);
U.putIntVolatile(obj, offset, newValue);
}
//延迟设置新值
public final void lazySet(T obj, int newValue) {
accessCheck(obj);
U.putOrderedInt(obj, offset, newValue);
}
//获取当前对象的值
public final int get(T obj) {
accessCheck(obj);
return U.getIntVolatile(obj, offset);
}
//设置并返回旧值
public final int getAndSet(T obj, int newValue) {
accessCheck(obj);
return U.getAndSetInt(obj, offset, newValue);
}
//旧值加delta并返回旧值
public final int getAndAdd(T obj, int delta) {
accessCheck(obj);
return U.getAndAddInt(obj, offset, delta);
}
//自增1返回旧值
public final int getAndIncrement(T obj) {
return getAndAdd(obj, 1);
}
//自减1返回旧值
public final int getAndDecrement(T obj) {
return getAndAdd(obj, -1);
}
//自增1返回新值
public final int incrementAndGet(T obj) {
return getAndAdd(obj, 1) + 1;
}
//自减1返回旧值
public final int decrementAndGet(T obj) {
return getAndAdd(obj, -1) - 1;
}
//当前值+delta并返回新值
public final int addAndGet(T obj, int delta) {
return getAndAdd(obj, delta) + delta;
}
}
}可以看到上面的的更新和实现大至都是类似，主要是这个AtomicIntegerFieldUpdater是带各种限制的，比如 必须是interger属性必须是public且被volatile修饰。
java.util.concurrent.atomic.DoubleAdder 源码学习
package java.util.concurrent.atomic;
import java.io.Serializable;
//累加器
public class DoubleAdder extends Striped64 implements Serializable {
private static final long serialVersionUID = 7249069246863182397L;
//无参构造方法
public DoubleAdder() {
}
//新增方法
public void add(double x) {
Cell[] as; long b, v; int m; Cell a;
//先判断是cells为空，如果是进行初始化;
//casBase对base的偏移量进行操作（没有竟争下）
if ((as = cells) != null ||
!casBase(b = base,
Double.doubleToRawLongBits
(Double.longBitsToDouble(b) + x))) {
boolean uncontended = true;
//as为空则进行初始化
if (as == null || (m = as.length - 1) < 0 ||
(a = as[getProbe() & m]) == null ||
!(uncontended = a.cas(v = a.value,
Double.doubleToRawLongBits
(Double.longBitsToDouble(v) + x))))
doubleAccumulate(x, null, uncontended);
}
}
//求合方法
public double sum() {
Cell[] as = cells; Cell a;
double sum = Double.longBitsToDouble(base);
//循环累加计算
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null)
sum += Double.longBitsToDouble(a.value);
}
}
return sum;
}
//重置（清空初始值）
public void reset() {
Cell[] as = cells; Cell a;
base = 0L; // relies on fact that double 0 must have same rep as long
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null)
a.value = 0L;
}
}
}
//返回该值为累加和（可能为旧值）
public double sumThenReset() {
Cell[] as = cells; Cell a;
double sum = Double.longBitsToDouble(base);
base = 0L;
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null) {
long v = a.value;
a.value = 0L;
sum += Double.longBitsToDouble(v);
}
}
}
return sum;
}
/**
* Returns the String representation of the {@link #sum}.
* @return the String representation of the {@link #sum}
*/
public String toString() {
return Double.toString(sum());
}
//返回当前的累加合
public double doubleValue() {
return sum();
}
//long类型的累加合
public long longValue() {
return (long)sum();
}
//整型的累加合
public int intValue() {
return (int)sum();
}
//浮点型的累加合
public float floatValue() {
return (float)sum();
}
//序列化
private static class SerializationProxy implements Serializable {
private static final long serialVersionUID = 7249069246863182397L;
/**
* The current value returned by sum().
* @serial
*/
private final double value;
SerializationProxy(DoubleAdder a) {
value = a.sum();
}
/**
* Returns a {@code DoubleAdder} object with initial state
* held by this proxy.
*
* @return a {@code DoubleAdder} object with initial state
* held by this proxy.
*/
private Object readResolve() {
DoubleAdder a = new DoubleAdder();
a.base = Double.doubleToRawLongBits(value);
return a;
}
}
//序列化
private Object writeReplace() {
return new SerializationProxy(this);
}
//读取字符流
private void readObject(java.io.ObjectInputStream s)
throws java.io.InvalidObjectException {
throw new java.io.InvalidObjectException("Proxy required");
}
}最后
其它的子类有兴趣的同学可以去深入了解，但是这里有一个特别要注意的，每个atomic类里面的实现大同小异的情况下，有些是特定版本解决特定问题而新建的比如aba问题我们一般要用AtomicStampedReference而不是直接用AtomicReference，所以在使用上，建议多看看别人踩过的坑。
参考文章：
https://zhuanlan.zhihu.com/p/494321482
https://www.bilibili.com/video/BV12Y411b7vL/?vd_source=7d0e42b081e08cb3cefaea55cc1fa8b7
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原始发表：2022-12-29，如有侵权请联系 cloudcommunity@tencent.com 删除
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