- 饿汉式
public class MySingleton1 {
private static MySingleton1 instance = new MySingleton1();
private MySingleton1(){}
//静态工厂方法
public static MySingleton1 getInstance(){
return instance;
}
}
- 懒汉式
public class MySingleton2 {
private static MySingleton2 instance = null;
private MySingleton2(){}
//静态工厂方法
public synchronized static MySingleton2 getInstance(){
if(instance == null){
instance = new MySingleton2();
}
return instance;
}
}
- 利用双重检查机制创建线程安全的单例对象
public class Singleton {
static volatile Singleton instance;//volatile可以避免cpu编译优化后带来的程序有序性问题
static Singleton getInstance(){
if (instance == null) {
synchronized(Singleton.class) {
if (instance == null)
instance = new Singleton();
}
}
return instance;
}
}
- 内部类实现单例
// 懒汉模式 内部类实现
public final class Singleton {
public List<String> list = null;// list 属性
private Singleton() {// 构造函数
list = new ArrayList<String>();
}
// 内部类实现
public static class InnerSingleton {
private static Singleton instance=new Singleton();// 自行创建实例
}
public static Singleton getInstance() {
return InnerSingleton.instance;// 返回内部类中的静态变量
}
}
- Spring对单例的实现
/** Cache of singleton objects: bean name --> bean instance 单例Bean的缓存池 */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(256);
/** Cache of early singleton objects: bean name --> bean instance执行了工厂方法生产出来的Bean */
private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16);
/** Cache of singleton factories: bean name --> ObjectFactory 单例Bean在创建之初过早的暴露出去的Factory*/
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<String, ObjectFactory<?>>(16);
//已经创建过的bean
private final Set<String> alreadyCreated =
Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(256));
//调用工厂产生bean(使用到spring的三个缓存)
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.调用上面的方法
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// 判断该Bean是否已经在创建
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
//标记已经创建
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
getBean(dep);
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
三级缓存
引出两个概念
- 循环依赖:环依赖就是循环引用，就是两个或多个Bean相互之间的持有对方，比如CircleA引用CircleB，CircleB引用CircleC，CircleC引用CircleA，则它们最终反映为一个环。此处不是循环调用，循环调用是方法之间的环调用
- 循环引用:循环调用是无法解决的，除非有终结条件，否则就是死循环，最终导致内存溢出错误。
- Spring容器循环依赖包括构造器循环依赖和setter循环依赖，那
Spring容器如何检测和解决循环依赖呢？
１．如何检测循环依赖
检测循环依赖相对比较容易，Bean在创建的时候可以给该Bean打标，如果递归调用回来发现正在创建中的话，即说明了循环依赖了。其实这点和Spring初始化的时候读配置文件涉及到import关键字会导致循环导入时的处理手法是一致的。
【DefaultSingletonBeanRegistry】
protected void beforeSingletonCreation(String beanName) {
if (!this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
上图是个单例Bean创建的实例，在创建之前先打标，然后在实例化的时候如果发现已经在创建了，即抛异常：
【DefaultSingletonBeanRegistry】
protected void beforeSingletonCreation(String beanName) {
AbstractBeanFactory】
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
２．如何检测循环依赖
循环调用是无法解决的，除非有终结条件，否则就是死循环，因此必须打破这个循环，具体做法如下：
假设场景如下，A->B->A
step1: 实例化A，并将未注入属性的A暴露出去，假设暴露给容器Wrap
step2: 开始为A注入属性，发现需要B，因此调用getBean(B)
step3:实例化B，并注入属性，当发现需要A的时候，从单例缓存中查找未果，继而从Wrap中查找，从而完成属性的注入
step4、递归完毕之后回到A的实例化过程，A将B注入成功，并注入A的其他属性值，自此即完成了循环依赖的注入。
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