ActionContext：
public class ActionContext implements Serializable {

static ThreadLocal<ActionContext> actionContext = new ThreadLocal<ActionContext>();//補充本質上就是一個ThreadLocal<T>將當前thread與entryT綁定

/**
 * 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;//其中

/**
 * 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;//將不同的域統一管理

如何統一管理：
//以setSession為例
public void setSession(Map<String, Object> session) {
put(SESSION, session);
}
public void put(String key, Object value) {
context.put(key, value);
}

OgnlValueStack(valueStack)
public class OgnlValueStack implements Serializable, ValueStack, ClearableValueStack, MemberAccessValueStack {

public static final String THROW_EXCEPTION_ON_FAILURE = OgnlValueStack.class.getName() + ".throwExceptionOnFailure";

private static final long serialVersionUID = 370737852934925530L;

private static final String MAP_IDENTIFIER_KEY = "com.opensymphony.xwork2.util.OgnlValueStack.MAP_IDENTIFIER_KEY";
private static final Logger LOG = LoggerFactory.getLogger(OgnlValueStack.class);

CompoundRoot root;//
transient Map<String, Object> context;//context(即為ActionContext):

如何保證ActionContext線程安全：
public static ActionContext getContext() {
//獲取當前線程綁定的ActionContext對象
return actionContext.get();
}

簡述： Thread, ThreadLocal, ThreadLocalMap關系
//Thread下有個ThreadLocal.ThreadLocalMap threadlocals
//從上可看出ThreadLocal中有個靜態類ThreadLocalMap..
獲取value(ThreadLocal綁定的object：value)簡述：
1.ThreadLocal中通過t.threadlocals獲取tm
2.而ThreadLocalMap又通過ThreadLocal查詢value
誤區：ThreadLocalMap并非為簡單的Map
而是一個size+1的數組..
public class ThreadLocal<T> {
static class ThreadLocalMap {

    /**
     * The table, resized as necessary.
     * table.length MUST always be a power of two.
     */
    private Entry[] table;

private void set(ThreadLocal key, Object value) {

        // We don't use a fast path as with get() because it is at
        // least as common to use set() to create new entries as
        // it is to replace existing ones, in which case, a fast
        // path would fail more often than not.

        Entry[] tab = table;
        int len = tab.length;
        int i = key.threadLocalHashCode & (len-1);

        for (Entry e = tab[i];
             e != null;
             e = tab[i = nextIndex(i, len)]) {
            ThreadLocal k = e.get();
            //如果已經存在就直接return
            if (k == key) {
                e.value = value;
                return;
            }

            if (k == null) {
                replaceStaleEntry(key, value, i);
                return;
            }
        }
        //不存在就new添加進數組或者
        tab[i] = new Entry(key, value);
        int sz = ++size;//后續步驟猜測：添加一個null進數組
        if (!cleanSomeSlots(i, sz) && sz >= threshold)
            rehash();

}