1、为什么需要TargetSource
TargetSource(目标源)是被代理的target(目标对象)实例的来源。
public interface TargetSource extends TargetClassAware {
//目标对象类型
Class<?> getTargetClass();
// 这个方法用户返回当前bean是否为静态的，比如常见的单例bean就是静态的，而prototype就是动态的，
// 这里这个方法的主要作用是，对于静态的bean，spring是会对其进行缓存的，在多次使用TargetSource
// 获取目标bean对象的时候，其获取的总是同一个对象，通过这种方式提高效率
boolean isStatic();
//获取目标对象
Object getTarget() throws Exception;
// Spring在完目标bean之后会调用这个方法释放目标bean对象，对于一些需要池化的对象，这个方法是必须
// 要实现的，这个方法默认不进行任何处理
void releaseTarget(Object target) throws Exception;
}
由这篇《Spring AOP的实现原理》可知TargetSource被用于获取当前MethodInvocation(方法调用)所需要的target(目标对象),这个target通过反射的方式被调用(如:method.invoke(target,args))。换句话说,proxy(代理对象)代理的不是target,而是TargetSource,这点非常重要!!!
那么问题来了:为什么SpringAOP代理不直接代理target,而需要通过代理TargetSource(target的来源,其内部持有target),间接代理target呢?
通常情况下,一个proxy(代理对象)只能代理一个target,每次方法调用的目标也是唯一固定的target。但是,如果让proxy代理TargetSource,可以使得每次方法调用的target实例都不同(当然也可以相同,这取决于TargetSource实现)。这种机制使得方法调用变得灵活,可以扩展出很多高级功能,如:target pool(目标对象池)、hot swap(运行时目标对象热替换),等等。
2、Spring内置的TargetSource
2.1、SingletonTargetSource
从这个目标源取得的目标对象是单例的，成员变量target缓存了目标对象，每次getTarget()都是返回这个对象。
public class SingletonTargetSource implements TargetSource, Serializable {
private static final long serialVersionUID = 9031246629662423738L;
private final Object target;
public SingletonTargetSource(Object target) {
Assert.notNull(target, "Target object must not be null");
this.target = target;
}
@Override
public Class<?> getTargetClass() {
return this.target.getClass();
}
@Override
public Object getTarget() {
return this.target;
}
@Override
public void releaseTarget(Object target) {
// nothing to do
}
@Override
public boolean isStatic() {
return true;
}
@Override
public boolean equals(Object other) {
if (this == other) {
return true;
}
if (!(other instanceof SingletonTargetSource)) {
return false;
}
SingletonTargetSource otherTargetSource = (SingletonTargetSource) other;
return this.target.equals(otherTargetSource.target);
}
@Override
public int hashCode() {
return this.target.hashCode();
}
@Override
public String toString() {
return "SingletonTargetSource for target object [" + ObjectUtils.identityToString(this.target) + "]";
}
}
2.2、PrototypeTargetSource
每次getTarget()将生成prototype类型的bean，即其生成的bean并不是单例的，因而使用这个类型的TargetSource时需要注意，封装的目标bean必须是prototype类型的。PrototypeTargetSource继承了AbstractBeanFactoryBasedTargetSource拥有了创建bean的能力。
public class PrototypeTargetSource extends AbstractPrototypeBasedTargetSource {
@Override
public Object getTarget() throws BeansException {
return newPrototypeInstance();
}
@Override
public void releaseTarget(Object target) {
destroyPrototypeInstance(target);
}
@Override
public String toString() {
return "PrototypeTargetSource for target bean with name '" + getTargetBeanName() + "'";
}
}
public abstract class AbstractPrototypeBasedTargetSource extends AbstractBeanFactoryBasedTargetSource {
@Override
public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
super.setBeanFactory(beanFactory);
// Check whether the target bean is defined as prototype.
if (!beanFactory.isPrototype(getTargetBeanName())) {
throw new BeanDefinitionStoreException(
"Cannot use prototype-based TargetSource against non-prototype bean with name '" +
getTargetBeanName() + "': instances would not be independent");
}
}
/**
* Subclasses should call this method to create a new prototype instance.
* @throws BeansException if bean creation failed
*/
protected Object newPrototypeInstance() throws BeansException {
if (logger.isDebugEnabled()) {
logger.debug("Creating new instance of bean '" + getTargetBeanName() + "'");
}
//使用容器创建一个bean，如果getTargetBeanName()是prototype的，则target目标对象也是prototype的
return getBeanFactory().getBean(getTargetBeanName());
}
/**
* Subclasses should call this method to destroy an obsolete prototype instance.
* @param target the bean instance to destroy
*/
protected void destroyPrototypeInstance(Object target) {
if (logger.isDebugEnabled()) {
logger.debug("Destroying instance of bean '" + getTargetBeanName() + "'");
}
if (getBeanFactory() instanceof ConfigurableBeanFactory) {
((ConfigurableBeanFactory) getBeanFactory()).destroyBean(getTargetBeanName(), target);
}
else if (target instanceof DisposableBean) {
try {
((DisposableBean) target).destroy();
}
catch (Throwable ex) {
logger.warn("Destroy method on bean with name '" + getTargetBeanName() + "' threw an exception", ex);
}
}
}
//---------------------------------------------------------------------
// Serialization support
//---------------------------------------------------------------------
private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
throw new NotSerializableException("A prototype-based TargetSource itself is not deserializable - " +
"just a disconnected SingletonTargetSource or EmptyTargetSource is");
}
/**
* Replaces this object with a SingletonTargetSource on serialization.
* Protected as otherwise it won't be invoked for subclasses.
* (The {@code writeReplace()} method must be visible to the class
* being serialized.)
* <p>With this implementation of this method, there is no need to mark
* non-serializable fields in this class or subclasses as transient.
*/
protected Object writeReplace() throws ObjectStreamException {
if (logger.isDebugEnabled()) {
logger.debug("Disconnecting TargetSource [" + this + "]");
}
try {
// Create disconnected SingletonTargetSource/EmptyTargetSource.
Object target = getTarget();
return (target != null ? new SingletonTargetSource(target) :
EmptyTargetSource.forClass(getTargetClass()));
}
catch (Exception ex) {
String msg = "Cannot get target for disconnecting TargetSource [" + this + "]";
logger.error(msg, ex);
throw new NotSerializableException(msg + ": " + ex);
}
}
}
可以看到，PrototypeTargetSource的生成prototype类型bean的方式主要是委托给BeanFactory进行的，因为BeanFactory自有一套生成prototype类型的bean的逻辑，因而PrototypeTargetSource也就具有生成prototype类型bean的能力，这也就是我们要生成的目标bean必须声明为prototype类型的原因。
2.3、CommonsPool2TargetSource
这里CommonsPool2TargetSource也就是池化的TargetSource，其基本具有平常所使用的“池”的概念的所有属性，比如：最小空闲数，最大空闲数，最大等待时间等等。实际上，CommonsPool2TargetSource的实现是将其委托给了ObjectPool进行，具体的也就是GenericObjectPool，其实现了ObjectPool接口。如下是CommonsPool2TargetSource的主要实现：
public class CommonsPool2TargetSource extends AbstractPoolingTargetSource implements PooledObjectFactory<Object> {
private int maxIdle = GenericObjectPoolConfig.DEFAULT_MAX_IDLE;
private int minIdle = GenericObjectPoolConfig.DEFAULT_MIN_IDLE;
private long maxWait = GenericObjectPoolConfig.DEFAULT_MAX_WAIT_MILLIS;
private long timeBetweenEvictionRunsMillis = GenericObjectPoolConfig.DEFAULT_TIME_BETWEEN_EVICTION_RUNS_MILLIS;
private long minEvictableIdleTimeMillis = GenericObjectPoolConfig.DEFAULT_MIN_EVICTABLE_IDLE_TIME_MILLIS;
private boolean blockWhenExhausted = GenericObjectPoolConfig.DEFAULT_BLOCK_WHEN_EXHAUSTED;
/**
* The Apache Commons {@code ObjectPool} used to pool target objects.
*/
@Nullable
private ObjectPool pool;
/**
* Create a CommonsPoolTargetSource with default settings.
* Default maximum size of the pool is 8.
* @see #setMaxSize
* @see GenericObjectPoolConfig#setMaxTotal
*/
public CommonsPool2TargetSource() {
setMaxSize(GenericObjectPoolConfig.DEFAULT_MAX_TOTAL);
}
/**
* Set the maximum number of idle objects in the pool.
* Default is 8.
* @see GenericObjectPool#setMaxIdle
*/
public void setMaxIdle(int maxIdle) {
this.maxIdle = maxIdle;
}
/**
* Return the maximum number of idle objects in the pool.
*/
public int getMaxIdle() {
return this.maxIdle;
}
/**
* Set the minimum number of idle objects in the pool.
* Default is 0.
* @see GenericObjectPool#setMinIdle
*/
public void setMinIdle(int minIdle) {
this.minIdle = minIdle;
}
/**
* Return the minimum number of idle objects in the pool.
*/
public int getMinIdle() {
return this.minIdle;
}
/**
* Set the maximum waiting time for fetching an object from the pool.
* Default is -1, waiting forever.
* @see GenericObjectPool#setMaxWaitMillis
*/
public void setMaxWait(long maxWait) {
this.maxWait = maxWait;
}
/**
* Return the maximum waiting time for fetching an object from the pool.
*/
public long getMaxWait() {
return this.maxWait;
}
/**
* Set the time between eviction runs that check idle objects whether
* they have been idle for too long or have become invalid.
* Default is -1, not performing any eviction.
* @see GenericObjectPool#setTimeBetweenEvictionRunsMillis
*/
public void setTimeBetweenEvictionRunsMillis(long timeBetweenEvictionRunsMillis) {
this.timeBetweenEvictionRunsMillis = timeBetweenEvictionRunsMillis;
}
/**
* Return the time between eviction runs that check idle objects.
*/
public long getTimeBetweenEvictionRunsMillis() {
return this.timeBetweenEvictionRunsMillis;
}
/**
* Set the minimum time that an idle object can sit in the pool before
* it becomes subject to eviction. Default is 1800000 (30 minutes).
* <p>Note that eviction runs need to be performed to take this
* setting into effect.
* @see #setTimeBetweenEvictionRunsMillis
* @see GenericObjectPool#setMinEvictableIdleTimeMillis
*/
public void setMinEvictableIdleTimeMillis(long minEvictableIdleTimeMillis) {
this.minEvictableIdleTimeMillis = minEvictableIdleTimeMillis;
}
/**
* Return the minimum time that an idle object can sit in the pool.
*/
public long getMinEvictableIdleTimeMillis() {
return this.minEvictableIdleTimeMillis;
}
/**
* Set whether the call should bock when the pool is exhausted.
*/
public void setBlockWhenExhausted(boolean blockWhenExhausted) {
this.blockWhenExhausted = blockWhenExhausted;
}
/**
* Specify if the call should block when the pool is exhausted.
*/
public boolean isBlockWhenExhausted() {
return this.blockWhenExhausted;
}
/**
* Creates and holds an ObjectPool instance.
* @see #createObjectPool()
*/
@Override
protected final void createPool() {
logger.debug("Creating Commons object pool");
this.pool = createObjectPool();
}
/**
* Subclasses can override this if they want to return a specific Commons pool.
* They should apply any configuration properties to the pool here.
* <p>Default is a GenericObjectPool instance with the given pool size.
* @return an empty Commons {@code ObjectPool}.
* @see GenericObjectPool
* @see #setMaxSize
*/
protected ObjectPool createObjectPool() {
GenericObjectPoolConfig config = new GenericObjectPoolConfig();
config.setMaxTotal(getMaxSize());
config.setMaxIdle(getMaxIdle());
config.setMinIdle(getMinIdle());
config.setMaxWaitMillis(getMaxWait());
config.setTimeBetweenEvictionRunsMillis(getTimeBetweenEvictionRunsMillis());
config.setMinEvictableIdleTimeMillis(getMinEvictableIdleTimeMillis());
config.setBlockWhenExhausted(isBlockWhenExhausted());
return new GenericObjectPool(this, config);
}
/**
* Borrows an object from the {@code ObjectPool}.
*/
@Override
public Object getTarget() throws Exception {
Assert.state(this.pool != null, "No Commons ObjectPool available");
return this.pool.borrowObject();
}
/**
* Returns the specified object to the underlying {@code ObjectPool}.
*/
@Override
public void releaseTarget(Object target) throws Exception {
if (this.pool != null) {
this.pool.returnObject(target);
}
}
@Override
public int getActiveCount() throws UnsupportedOperationException {
return (this.pool != null ? this.pool.getNumActive() : 0);
}
@Override
public int getIdleCount() throws UnsupportedOperationException {
return (this.pool != null ? this.pool.getNumIdle() : 0);
}
/**
* Closes the underlying {@code ObjectPool} when destroying this object.
*/
@Override
public void destroy() throws Exception {
if (this.pool != null) {
logger.debug("Closing Commons ObjectPool");
this.pool.close();
}
}
//----------------------------------------------------------------------------
// Implementation of org.apache.commons.pool2.PooledObjectFactory interface
//----------------------------------------------------------------------------
@Override
public PooledObject<Object> makeObject() throws Exception {
return new DefaultPooledObject<>(newPrototypeInstance());
}
@Override
public void destroyObject(PooledObject<Object> p) throws Exception {
destroyPrototypeInstance(p.getObject());
}
@Override
public boolean validateObject(PooledObject<Object> p) {
return true;
}
@Override
public void activateObject(PooledObject<Object> p) throws Exception {
}
@Override
public void passivateObject(PooledObject<Object> p) throws Exception {
}
}
2.4、ThreadLocalTargetSource
ThreadLocalTargetSource也就是和线程绑定的TargetSource，可以理解，其底层实现必然使用的是ThreadLocal。既然使用了ThreadLocal，也就是说我们需要注意两个问题：
- 目标对象必须声明为prototype类型，因为每个线程都会持有一个不一样的对象；
- 目标对象必须是无状态的，因为目标对象是和当前线程绑定的，而Spring是使用的线程池处理的请求，因而每个线程可能处理不同的请求，因而为了避免造成问题，目标对象必须是无状态的。
public class ThreadLocalTargetSource extends AbstractPrototypeBasedTargetSource
implements ThreadLocalTargetSourceStats, DisposableBean {
/**
* ThreadLocal holding the target associated with the current
* thread. Unlike most ThreadLocals, which are static, this variable
* is meant to be per thread per instance of the ThreadLocalTargetSource class.
*/
private final ThreadLocal<Object> targetInThread =
new NamedThreadLocal<>("Thread-local instance of bean '" + getTargetBeanName() + "'");
/**
* Set of managed targets, enabling us to keep track of the targets we've created.
*/
private final Set<Object> targetSet = new HashSet<>();
private int invocationCount;
private int hitCount;
@Override
public Object getTarget() throws BeansException {
++this.invocationCount;
Object target = this.targetInThread.get();
if (target == null) {
if (logger.isDebugEnabled()) {
logger.debug("No target for prototype '" + getTargetBeanName() + "' bound to thread: " +
"creating one and binding it to thread '" + Thread.currentThread().getName() + "'");
}
// Associate target with ThreadLocal.
target = newPrototypeInstance();
this.targetInThread.set(target);
synchronized (this.targetSet) {
this.targetSet.add(target);
}
}
else {
++this.hitCount;
}
return target;
}
@Override
public void destroy() {
logger.debug("Destroying ThreadLocalTargetSource bindings");
synchronized (this.targetSet) {
for (Object target : this.targetSet) {
destroyPrototypeInstance(target);
}
this.targetSet.clear();
}
// Clear ThreadLocal, just in case.
this.targetInThread.remove();
}
@Override
public int getInvocationCount() {
return this.invocationCount;
}
@Override
public int getHitCount() {
return this.hitCount;
}
@Override
public int getObjectCount() {
synchronized (this.targetSet) {
return this.targetSet.size();
}
}
public IntroductionAdvisor getStatsMixin() {
DelegatingIntroductionInterceptor dii = new DelegatingIntroductionInterceptor(this);
return new DefaultIntroductionAdvisor(dii, ThreadLocalTargetSourceStats.class);
}
}
这里ThreadLocalTargetSource主要集成了AbstractPrototypeBasedTargetSource和DisposableBean。关于AbstractPrototypeBasedTargetSource前面已经讲过了，读者可以到前面翻看；而DisposableBean的作用主要是提供一个方法，以供给Spring在销毁当前对象的时候调用。也就是说Spring在销毁当前TargetSource对象的时候会首先销毁其生成的各个目标对象。这里需要注意的是，TargetSource和生成的目标对象是两个对象，前面讲的TargetSouce都是单例的，只是生成的目标对象可能是单例的，也可能是多例的。
2.5、HotSwappableTargetSource
HotSwappableTargetSource使用户可以以线程安全的方式切换目标对象,提供所谓的热交换功能。这个特性是很有用的,尽管它的开启需要AOP应用进行显式的配置,但配置并不复杂,在使用时,只需要把 HotSwappableargetSource配置到ProxyFactoryBean的Target属性就可以了,在需要更换真正的目标对象时,调用HotSwappableTargetSource的swap方法就可以完成。由此可见,对HotSwappableTargetSource的热交换功能的使用,是需要触发swap方法调用的。这个swap方法的实现很简单,它完成 target对象的替换,也就是说,它使用新的 target对象来替换原有的 target对象。为了保证线程安全,需要把这个替换方法设为 synchronized方法。
public class HotSwappableTargetSource implements TargetSource, Serializable {
/** use serialVersionUID from Spring 1.2 for interoperability. */
private static final long serialVersionUID = 7497929212653839187L;
/** The current target object. */
private Object target;
/**
* Create a new HotSwappableTargetSource with the given initial target object.
* @param initialTarget the initial target object
*/
public HotSwappableTargetSource(Object initialTarget) {
Assert.notNull(initialTarget, "Target object must not be null");
this.target = initialTarget;
}
/**
* Return the type of the current target object.
* <p>The returned type should usually be constant across all target objects.
*/
@Override
public synchronized Class<?> getTargetClass() {
return this.target.getClass();
}
@Override
public final boolean isStatic() {
return false;
}
@Override
public synchronized Object getTarget() {
return this.target;
}
@Override
public void releaseTarget(Object target) {
// nothing to do
}
/**
* Swap the target, returning the old target object.
* @param newTarget the new target object
* @return the old target object
* @throws IllegalArgumentException if the new target is invalid
*/
public synchronized Object swap(Object newTarget) throws IllegalArgumentException {
Assert.notNull(newTarget, "Target object must not be null");
Object old = this.target;
this.target = newTarget;
return old;
}
/**
* Two HotSwappableTargetSources are equal if the current target
* objects are equal.
*/
@Override
public boolean equals(Object other) {
return (this == other || (other instanceof HotSwappableTargetSource &&
this.target.equals(((HotSwappableTargetSource) other).target)));
}
@Override
public int hashCode() {
return HotSwappableTargetSource.class.hashCode();
}
@Override
public String toString() {
return "HotSwappableTargetSource for target: " + this.target;
}
}
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