Struts2系列：（16）ActionContext

本文解决下面几个问题：
1、ActionContext是什么？
2、如何获取到ActionContext对象？
3、ActionContext的核心结构是什么？
4、ActionContext定义的一些常量
5、ActionContext的完整源代码

1、ActionContext是什么？ActionContext是Action运行的上下文环境。本质上来说，每一个context就是对象的容器。The ActionContext is the context in which an Action is executed. Each context is basically a container of objects an action needs for execution like the session, parameters, locale, etc.
2、如何获取到ActionContext对象？

ActionContext context = ActionContext.getContext();

3、ActionContext的核心结构是什么？ActionContext的核心结构是维护了一个Map<String, Object>类型的context变量。数据的存储和读取都是通过context对象来实现了。
下面是一个简略版本的ActionContext类的实现：

/**
* ActionContext是Action运行的上下文环境。本质上来说，每一个context就是对象的容器。
* The ActionContext is the context in which an Action is executed. Each context is basically a
* container of objects an action needs for execution like the session, parameters, locale, etc.
*
* Action是thread local，是线程安全的。
* The ActionContext is thread local which means that values stored in the ActionContext are
* unique per thread. The benefit of this is you don't need to worry about
* a user specific action context, you just get it:
*
*           ActionContext context = ActionContext.getContext();
*
* Finally, because of the thread local usage you don't need to worry about making your actions thread safe.
*
* @author Patrick Lightbody
* @author Bill Lynch (docs)
*/
public class ActionContext implements Serializable {

/**
* 1、为了线程安全，而做一些准备工作
**/
static ThreadLocal<ActionContext> actionContext = new ThreadLocal<ActionContext>();

//1.1、设置或获取ActionContext对象
public static void setContext(ActionContext context) {
actionContext.set(context);
}

public static ActionContext getContext() {
return actionContext.get();
}

/**
* 2、ActionContext的核心存储结构：Map<String, Object> context
**/
private Map<String, Object> context;

public ActionContext(Map<String, Object> context) {
this.context = context;
}

//2.1、设置和获取context对象
public void setContextMap(Map<String, Object> contextMap) {
getContext().context = contextMap;
}

public Map<String, Object> getContextMap() {
return context;
}

//2.2、向context对象中存储数据和从context对象中读取数据
public Object get(String key) {
return context.get(key);
}

public void put(String key, Object value) {
context.put(key, value);
}
}

4、ActionContext定义的一些常量4.1、ACTION_NAME常量

//Constant for the name of the action being executed.
public static final String ACTION_NAME = "com.opensymphony.xwork2.ActionContext.name";

/**
* Sets the name of the current Action in the ActionContext.
*
* @param name the name of the current action.
*/
public void setName(String name) {
put(ACTION_NAME, name);
}

/**
* Gets the name of the current Action.
*
* @return the name of the current action.
*/
public String getName() {
return (String) get(ACTION_NAME);
}

4.2、VALUE_STACK常量

//Constant for the {@link com.opensymphony.xwork2.util.ValueStack OGNL value stack}.
public static final String VALUE_STACK = ValueStack.VALUE_STACK;

/**
* Sets the OGNL value stack.
*
* @param stack the OGNL value stack.
*/
public void setValueStack(ValueStack stack) {
put(VALUE_STACK, stack);
}

/**
* Gets the OGNL value stack.
*
* @return the OGNL value stack.
*/
public ValueStack getValueStack() {
return (ValueStack) get(VALUE_STACK);
}

4.3、ACTION_INVOCATION常量

//Constant for the action's {@link com.opensymphony.xwork2.ActionInvocation invocation} context.
public static final String ACTION_INVOCATION = "com.opensymphony.xwork2.ActionContext.actionInvocation";

/**
* Sets the action invocation (the execution state).
*
* @param actionInvocation the action execution state.
*/
public void setActionInvocation(ActionInvocation actionInvocation) {
put(ACTION_INVOCATION, actionInvocation);
}

/**
* Gets the action invocation (the execution state).
*
* @return the action invocation (the execution state).
*/
public ActionInvocation getActionInvocation() {
return (ActionInvocation) get(ACTION_INVOCATION);
}

4.4、APPLICATION常量

//Constant for the action's application context.
public static final String APPLICATION = "com.opensymphony.xwork2.ActionContext.application";

/**
* Sets the action's application context.
*
* @param application the action's application context.
*/
public void setApplication(Map<String, Object> application) {
put(APPLICATION, application);
}

/**
* Returns a Map of the ServletContext when in a servlet environment or a generic application level Map otherwise.
*
* @return a Map of ServletContext or generic application level Map
*/
public Map<String, Object> getApplication() {
return (Map<String, Object>) get(APPLICATION);
}

4.5、SESSION常量

//Constant for the action's session.
public static final String SESSION = "com.opensymphony.xwork2.ActionContext.session";

/**
* Sets a map of action session values.
*
* @param session  the session values.
*/
public void setSession(Map<String, Object> session) {
put(SESSION, session);
}

/**
* Gets the Map of HttpSession values when in a servlet environment or a generic session map otherwise.
*
* @return the Map of HttpSession values when in a servlet environment or a generic session map otherwise.
*/
public Map<String, Object> getSession() {
return (Map<String, Object>) get(SESSION);
}

4.6、PARAMETERS常量

//Constant for the action's parameters.
public static final String PARAMETERS = "com.opensymphony.xwork2.ActionContext.parameters";

/**
* Sets the action parameters.
*
* @param parameters the parameters for the current action.
*/
public void setParameters(Map<String, Object> parameters) {
put(PARAMETERS, parameters);
}

/**
* Returns a Map of the HttpServletRequest parameters when in a servlet environment or a generic Map of
* parameters otherwise.
*
* @return a Map of HttpServletRequest parameters or a multipart map when in a servlet environment, or a
*         generic Map of parameters otherwise.
*/
public Map<String, Object> getParameters() {
return (Map<String, Object>) get(PARAMETERS);
}

4.7、LOCALE常量

//Constant for the action's locale.
public static final String LOCALE = "com.opensymphony.xwork2.ActionContext.locale";

/**
* Sets the Locale for the current action.
*
* @param locale the Locale for the current action.
*/
public void setLocale(Locale locale) {
put(LOCALE, locale);
}

/**
* Gets the Locale of the current action. If no locale was ever specified the platform's
* {@link java.util.Locale#getDefault() default locale} is used.
*
* @return the Locale of the current action.
*/
public Locale getLocale() {
Locale locale = (Locale) get(LOCALE);

if (locale == null) {
locale = Locale.getDefault();
setLocale(locale);
}

return locale;
}

5、ActionContext的完整源代码

package com.opensymphony.xwork2;

import com.opensymphony.xwork2.inject.Container;
import com.opensymphony.xwork2.util.ValueStack;

import java.io.Serializable;
import java.util.HashMap;
import java.util.Locale;
import java.util.Map;

/**
* The ActionContext is the context in which an {@link Action} is executed. Each context is basically a
* container of objects an action needs for execution like the session, parameters, locale, etc. <p>
* <p/>
* The ActionContext is thread local which means that values stored in the ActionContext are
* unique per thread. See the {@link ThreadLocal} class for more information. The benefit of
* this is you don't need to worry about a user specific action context, you just get it:
* <p/>
* <ul><code>ActionContext context = ActionContext.getContext();</code></ul>
* <p/>
* Finally, because of the thread local usage you don't need to worry about making your actions thread safe.
*
* @author Patrick Lightbody
* @author Bill Lynch (docs)
*/
public class ActionContext implements Serializable {

static ThreadLocal<ActionContext> actionContext = new ThreadLocal<ActionContext>();

/**
* Constant for the name of the action being executed.
*/
public static final String ACTION_NAME = "com.opensymphony.xwork2.ActionContext.name";

/**
* Constant for the {@link com.opensymphony.xwork2.util.ValueStack OGNL value stack}.
*/
public static final String VALUE_STACK = ValueStack.VALUE_STACK;

/**
* Constant for the action's session.
*/
public static final String SESSION = "com.opensymphony.xwork2.ActionContext.session";

/**
* Constant for the action's application context.
*/
public static final String APPLICATION = "com.opensymphony.xwork2.ActionContext.application";

/**
* Constant for the action's parameters.
*/
public static final String PARAMETERS = "com.opensymphony.xwork2.ActionContext.parameters";

/**
* Constant for the action's locale.
*/
public static final String LOCALE = "com.opensymphony.xwork2.ActionContext.locale";

/**
* Constant for the action's type converter.
*/
public static final String TYPE_CONVERTER = "com.opensymphony.xwork2.ActionContext.typeConverter";

/**
* Constant for the action's {@link com.opensymphony.xwork2.ActionInvocation invocation} context.
*/
public static final String ACTION_INVOCATION = "com.opensymphony.xwork2.ActionContext.actionInvocation";

/**
* Constant for the map of type conversion errors.
*/
public static final String CONVERSION_ERRORS = "com.opensymphony.xwork2.ActionContext.conversionErrors";

/**
* Constant for the container
*/
public static final String CONTAINER = "com.opensymphony.xwork2.ActionContext.container";

private Map<String, Object> context;

/**
* Creates a new ActionContext initialized with another context.
*
* @param context a context map.
*/
public ActionContext(Map<String, Object> context) {
this.context = context;
}

/**
* Sets the action invocation (the execution state).
*
* @param actionInvocation the action execution state.
*/
public void setActionInvocation(ActionInvocation actionInvocation) {
put(ACTION_INVOCATION, actionInvocation);
}

/**
* Gets the action invocation (the execution state).
*
* @return the action invocation (the execution state).
*/
public ActionInvocation getActionInvocation() {
return (ActionInvocation) get(ACTION_INVOCATION);
}

/**
* Sets the action's application context.
*
* @param application the action's application context.
*/
public void setApplication(Map<String, Object> application) {
put(APPLICATION, application);
}

/**
* Returns a Map of the ServletContext when in a servlet environment or a generic application level Map otherwise.
*
* @return a Map of ServletContext or generic application level Map
*/
public Map<String, Object> getApplication() {
return (Map<String, Object>) get(APPLICATION);
}

/**
* Sets the action context for the current thread.
*
* @param context the action context.
*/
public static void setContext(ActionContext context) {
actionContext.set(context);
}

/**
* Returns the ActionContext specific to the current thread.
*
* @return the ActionContext for the current thread, is never <tt>null</tt>.
*/
public static ActionContext getContext() {
return actionContext.get();
}

/**
* Sets the action's context map.
*
* @param contextMap the context map.
*/
public void setContextMap(Map<String, Object> contextMap) {
getContext().context = contextMap;
}

/**
* Gets the context map.
*
* @return the context map.
*/
public Map<String, Object> getContextMap() {
return context;
}

/**
* Sets conversion errors which occurred when executing the action.
*
* @param conversionErrors a Map of errors which occurred when executing the action.
*/
public void setConversionErrors(Map<String, Object> conversionErrors) {
put(CONVERSION_ERRORS, conversionErrors);
}

/**
* Gets the map of conversion errors which occurred when executing the action.
*
* @return the map of conversion errors which occurred when executing the action or an empty map if
*         there were no errors.
*/
public Map<String, Object> getConversionErrors() {
Map<String, Object> errors = (Map) get(CONVERSION_ERRORS);

if (errors == null) {
errors = new HashMap<String, Object>();
setConversionErrors(errors);
}

return errors;
}

/**
* Sets the Locale for the current action.
*
* @param locale the Locale for the current action.
*/
public void setLocale(Locale locale) {
put(LOCALE, locale);
}

/**
* Gets the Locale of the current action. If no locale was ever specified the platform's
* {@link java.util.Locale#getDefault() default locale} is used.
*
* @return the Locale of the current action.
*/
public Locale getLocale() {
Locale locale = (Locale) get(LOCALE);

if (locale == null) {
locale = Locale.getDefault();
setLocale(locale);
}

return locale;
}

/**
* Sets the name of the current Action in the ActionContext.
*
* @param name the name of the current action.
*/
public void setName(String name) {
put(ACTION_NAME, name);
}

/**
* Gets the name of the current Action.
*
* @return the name of the current action.
*/
public String getName() {
return (String) get(ACTION_NAME);
}

/**
* Sets the action parameters.
*
* @param parameters the parameters for the current action.
*/
public void setParameters(Map<String, Object> parameters) {
put(PARAMETERS, parameters);
}

/**
* Returns a Map of the HttpServletRequest parameters when in a servlet environment or a generic Map of
* parameters otherwise.
*
* @return a Map of HttpServletRequest parameters or a multipart map when in a servlet environment, or a
*         generic Map of parameters otherwise.
*/
public Map<String, Object> getParameters() {
return (Map<String, Object>) get(PARAMETERS);
}

/**
* Sets a map of action session values.
*
* @param session  the session values.
*/
public void setSession(Map<String, Object> session) {
put(SESSION, session);
}

/**
* Gets the Map of HttpSession values when in a servlet environment or a generic session map otherwise.
*
* @return the Map of HttpSession values when in a servlet environment or a generic session map otherwise.
*/
public Map<String, Object> getSession() {
return (Map<String, Object>) get(SESSION);
}

/**
* Sets the OGNL value stack.
*
* @param stack the OGNL value stack.
*/
public void setValueStack(ValueStack stack) {
put(VALUE_STACK, stack);
}

/**
* Gets the OGNL value stack.
*
* @return the OGNL value stack.
*/
public ValueStack getValueStack() {
return (ValueStack) get(VALUE_STACK);
}

/**
* Gets the container for this request
*
* @param cont The container
*/
public void setContainer(Container cont) {
put(CONTAINER, cont);
}

/**
* Sets the container for this request
*
* @return The container
*/
public Container getContainer() {
return (Container) get(CONTAINER);
}

public <T> T getInstance(Class<T> type) {
Container cont = getContainer();
if (cont != null) {
return cont.getInstance(type);
} else {
throw new XWorkException("Cannot find an initialized container for this request.");
}
}

/**
* Returns a value that is stored in the current ActionContext by doing a lookup using the value's key.
*
* @param key the key used to find the value.
* @return the value that was found using the key or <tt>null</tt> if the key was not found.
*/
public Object get(String key) {
return context.get(key);
}

/**
* Stores a value in the current ActionContext. The value can be looked up using the key.
*
* @param key   the key of the value.
* @param value the value to be stored.
*/
public void put(String key, Object value) {
context.put(key, value);
}
}