Struts2中的ValueStack詳解

ValueStack簡述

valueStack是一個接口，在struts2中使用OGNL表達式實際上是使用實現了ValueStack接口的類OgnlValueStack.它是ValueStack的默認實現類. 首先看一下，這個接口是怎麼定義的。

package com.opensymphony.xwork2.util;

import java.util.Map;
public interface ValueStack {

    public static final String VALUE_STACK = "com.opensymphony.xwork2.util.ValueStack.ValueStack";

    public static final String REPORT_ERRORS_ON_NO_PROP = "com.opensymphony.xwork2.util.ValueStack.ReportErrorsOnNoProp";

    public abstract Map<String, Object> getContext();

    /**
     * Sets the default type to convert to if no type is provided when getting a value.
     *
     * @param defaultType the new default type
     */
    public abstract void setDefaultType(Class defaultType);

    /**
     * Set a override map containing <code>key -> values</code> that takes precedent when doing find operations on the ValueStack.
     * <p/>
     * See the unit test for ValueStackTest for examples.
     *
     * @param overrides  overrides map.
     */
    public abstract void setExprOverrides(Map<Object, Object> overrides);

    /**
     * Gets the override map if anyone exists.
     *
     * @return the override map, <tt>null</tt> if not set.
     */
    public abstract Map<Object, Object> getExprOverrides();

    /**
     * Get the CompoundRoot which holds the objects pushed onto the stack
     *
     * @return the root
     */
    public abstract CompoundRoot getRoot();

    /**
     * Attempts to set a property on a bean in the stack with the given expression using the default search order.
     *
     * @param expr  the expression defining the path to the property to be set.
     * @param value the value to be set into the named property
     */
    public abstract void setValue(String expr, Object value);

    /**
     * Attempts to set a property on a bean in the stack with the given expression using the default search order.
     * N.B.: unlike #setValue(String,Object) it doesn't allow eval expression.
     * @param expr  the expression defining the path to the property to be set.
     * @param value the value to be set into the named property
     */
    void setParameter(String expr, Object value);

    /**
     * 根據順序查找到表達式對應的值並修改
     *
     * @param expr                    表達式
     * @param value                   值
     * @param throwExceptionOnFailure 如果沒有找到拋出的異常
     */
    public abstract void setValue(String expr, Object value, boolean throwExceptionOnFailure);

    public abstract String findString(String expr);
    public abstract String findString(String expr, boolean throwExceptionOnFailure);

    /**
     * 按照默認的順序根據表達式獲取值
     */
    public abstract Object findValue(String expr);
    public abstract Object findValue(String expr, boolean throwExceptionOnFailure);
    public abstract Object findValue(String expr, Class asType);
    public abstract Object findValue(String expr, Class asType,  boolean throwExceptionOnFailure);

    /**
     * 獲取棧頂元素，不改變棧
     */
    public abstract Object peek();

    /**
     * 獲取棧頂元素並將它移除出棧
     */
    public abstract Object pop();

    /**
     * 將這個值放到棧頂
     */
    public abstract void push(Object o);

    /**
     * 修改值棧中鍵爲key的值
     */
    public abstract void set(String key, Object o);

    /**
     * 獲取棧中的對象的數量
     */
    public abstract int size();

}

可以看到，這個接口定義了許多的抽象方法以及方法。就是對值棧中的數據進行相應的操作。

它的默認實現類是OgnlValueStack，查看一下OgnlValueStack的源代碼：

package com.opensymphony.xwork2.ognl;

import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.TextProvider;
import com.opensymphony.xwork2.XWorkConstants;
import com.opensymphony.xwork2.XWorkException;
import com.opensymphony.xwork2.conversion.impl.XWorkConverter;
import com.opensymphony.xwork2.inject.Container;
import com.opensymphony.xwork2.inject.Inject;
import com.opensymphony.xwork2.ognl.accessor.CompoundRootAccessor;
import com.opensymphony.xwork2.util.ClearableValueStack;
import com.opensymphony.xwork2.util.CompoundRoot;
import com.opensymphony.xwork2.util.MemberAccessValueStack;
import com.opensymphony.xwork2.util.ValueStack;
import com.opensymphony.xwork2.util.logging.Logger;
import com.opensymphony.xwork2.util.logging.LoggerFactory;
import com.opensymphony.xwork2.util.logging.LoggerUtils;
import com.opensymphony.xwork2.util.reflection.ReflectionContextState;
import ognl.*;

import java.io.Serializable;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.regex.Pattern;

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繼承了ArraryList，提供了額外的方法：push()和pop()方法，所以它相當於棧結構
    CompoundRoot root;
    transient Map<String, Object> context;
    Class defaultType;
    Map<Object, Object> overrides;
    transient OgnlUtil ognlUtil;
    transient SecurityMemberAccess securityMemberAccess;

    private boolean devMode;
    private boolean logMissingProperties;

    protected OgnlValueStack(XWorkConverter xworkConverter, CompoundRootAccessor accessor, TextProvider prov, boolean allowStaticAccess) {
        setRoot(xworkConverter, accessor, new CompoundRoot(), allowStaticAccess);
        push(prov);
    }

    protected OgnlValueStack(ValueStack vs, XWorkConverter xworkConverter, CompoundRootAccessor accessor, boolean allowStaticAccess) {
        setRoot(xworkConverter, accessor, new CompoundRoot(vs.getRoot()), allowStaticAccess);
    }

    @Inject
    public void setOgnlUtil(OgnlUtil ognlUtil) {
        this.ognlUtil = ognlUtil;
    }

    protected void setRoot(XWorkConverter xworkConverter, CompoundRootAccessor accessor, CompoundRoot compoundRoot,
                           boolean allowStaticMethodAccess) {
        this.root = compoundRoot;
        this.securityMemberAccess = new SecurityMemberAccess(allowStaticMethodAccess);
        this.context = Ognl.createDefaultContext(this.root, accessor, new OgnlTypeConverterWrapper(xworkConverter), securityMemberAccess);
        context.put(VALUE_STACK, this);
        Ognl.setClassResolver(context, accessor);
        ((OgnlContext) context).setTraceEvaluations(false);
        ((OgnlContext) context).setKeepLastEvaluation(false);
    }

    @Inject(XWorkConstants.DEV_MODE)
    public void setDevMode(String mode) {
        devMode = "true".equalsIgnoreCase(mode);
    }

    @Inject(value = "logMissingProperties", required = false)
    public void setLogMissingProperties(String logMissingProperties) {
        this.logMissingProperties = "true".equalsIgnoreCase(logMissingProperties);
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#getContext()
     */
    public Map<String, Object> getContext() {
        return context;
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#setDefaultType(java.lang.Class)
     */
    public void setDefaultType(Class defaultType) {
        this.defaultType = defaultType;
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#setExprOverrides(java.util.Map)
     */
    public void setExprOverrides(Map<Object, Object> overrides) {
        if (this.overrides == null) {
            this.overrides = overrides;
        } else {
            this.overrides.putAll(overrides);
        }
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#getExprOverrides()
     */
    public Map<Object, Object> getExprOverrides() {
        return this.overrides;
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#getRoot()
     */
    public CompoundRoot getRoot() {
        return root;
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#setParameter(String, Object)
     */
    public void setParameter(String expr, Object value) {
        setValue(expr, value, devMode, false);
    }

    /**

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#setValue(java.lang.String, java.lang.Object)
     */
    public void setValue(String expr, Object value) {
        setValue(expr, value, devMode);
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#setValue(java.lang.String, java.lang.Object, boolean)
     */
    public void setValue(String expr, Object value, boolean throwExceptionOnFailure) {
        setValue(expr, value, throwExceptionOnFailure, true);
    }

    private void setValue(String expr, Object value, boolean throwExceptionOnFailure, boolean evalExpression) {
        Map<String, Object> context = getContext();
        try {
            trySetValue(expr, value, throwExceptionOnFailure, context, evalExpression);
        } catch (OgnlException e) {
            handleOgnlException(expr, value, throwExceptionOnFailure, e);
        } catch (RuntimeException re) { //XW-281
            handleRuntimeException(expr, value, throwExceptionOnFailure, re);
        } finally {
            cleanUpContext(context);
        }
    }

    private void trySetValue(String expr, Object value, boolean throwExceptionOnFailure, Map<String, Object> context, boolean evalExpression) throws OgnlException {
        context.put(XWorkConverter.CONVERSION_PROPERTY_FULLNAME, expr);
        context.put(REPORT_ERRORS_ON_NO_PROP, (throwExceptionOnFailure) ? Boolean.TRUE : Boolean.FALSE);
        ognlUtil.setValue(expr, context, root, value, evalExpression);
    }

    private void cleanUpContext(Map<String, Object> context) {
        ReflectionContextState.clear(context);
        context.remove(XWorkConverter.CONVERSION_PROPERTY_FULLNAME);
        context.remove(REPORT_ERRORS_ON_NO_PROP);
    }

    private void handleRuntimeException(String expr, Object value, boolean throwExceptionOnFailure, RuntimeException re) {
        if (throwExceptionOnFailure) {
            String message = ErrorMessageBuilder.create()
                    .errorSettingExpressionWithValue(expr, value)
                    .build();
            throw new XWorkException(message, re);
        } else {
            if (LOG.isWarnEnabled()) {
                LOG.warn("Error setting value", re);
            }
        }
    }

    private void handleOgnlException(String expr, Object value, boolean throwExceptionOnFailure, OgnlException e) {
        String msg = "Error setting expression '" + expr + "' with value '" + value + "'";
        if (LOG.isWarnEnabled()) {
            LOG.warn(msg, e);
        }
        if (throwExceptionOnFailure) {
            throw new XWorkException(msg, e);
        }
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#findString(java.lang.String)
     */
    public String findString(String expr) {
        return (String) findValue(expr, String.class);
    }

    public String findString(String expr, boolean throwExceptionOnFailure) {
        return (String) findValue(expr, String.class, throwExceptionOnFailure);
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#findValue(java.lang.String)
     */
    public Object findValue(String expr, boolean throwExceptionOnFailure) {
        try {
            setupExceptionOnFailure(throwExceptionOnFailure);
            return tryFindValueWhenExpressionIsNotNull(expr);
        } catch (OgnlException e) {
            return handleOgnlException(expr, throwExceptionOnFailure, e);
        } catch (Exception e) {
            return handleOtherException(expr, throwExceptionOnFailure, e);
        } finally {
            ReflectionContextState.clear(context);
        }
    }

    private void setupExceptionOnFailure(boolean throwExceptionOnFailure) {
        if (throwExceptionOnFailure) {
            context.put(THROW_EXCEPTION_ON_FAILURE, true);
        }
    }

    private Object tryFindValueWhenExpressionIsNotNull(String expr) throws OgnlException {
        if (expr == null) {
            return null;
        }
        return tryFindValue(expr);
    }

    private Object handleOtherException(String expr, boolean throwExceptionOnFailure, Exception e) {
        logLookupFailure(expr, e);

        if (throwExceptionOnFailure)
            throw new XWorkException(e);

        return findInContext(expr);
    }

    private Object tryFindValue(String expr) throws OgnlException {
        Object value;
        expr = lookupForOverrides(expr);
        if (defaultType != null) {
            value = findValue(expr, defaultType);
        } else {
            value = getValueUsingOgnl(expr);
            if (value == null) {
                value = findInContext(expr);
            }
        }
        return value;
    }

    private String lookupForOverrides(String expr) {
        if ((overrides != null) && overrides.containsKey(expr)) {
            expr = (String) overrides.get(expr);
        }
        return expr;
    }

    private Object getValueUsingOgnl(String expr) throws OgnlException {
        try {
            return ognlUtil.getValue(expr, context, root);
        } finally {
            context.remove(THROW_EXCEPTION_ON_FAILURE);
        }
    }

    public Object findValue(String expr) {
        return findValue(expr, false);
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#findValue(java.lang.String, java.lang.Class)
     */
    public Object findValue(String expr, Class asType, boolean throwExceptionOnFailure) {
        try {
            setupExceptionOnFailure(throwExceptionOnFailure);
            return tryFindValueWhenExpressionIsNotNull(expr, asType);
        } catch (OgnlException e) {
            return handleOgnlException(expr, throwExceptionOnFailure, e);
        } catch (Exception e) {
            return handleOtherException(expr, throwExceptionOnFailure, e);
        } finally {
            ReflectionContextState.clear(context);
        }
    }

    private Object tryFindValueWhenExpressionIsNotNull(String expr, Class asType) throws OgnlException {
        if (expr == null) {
            return null;
        }
        return tryFindValue(expr, asType);
    }

    private Object handleOgnlException(String expr, boolean throwExceptionOnFailure, OgnlException e) {
        Object ret = findInContext(expr);
        if (ret == null) {
            if (shouldLogNoSuchPropertyWarning(e)) {
                LOG.warn("Could not find property [" + ((NoSuchPropertyException) e).getName() + "]");
            }
            if (throwExceptionOnFailure) {
                throw new XWorkException(e);
            }
        }
        return ret;
    }

    private boolean shouldLogNoSuchPropertyWarning(OgnlException e) {
        return e instanceof NoSuchPropertyException && devMode && logMissingProperties;
    }

    private Object tryFindValue(String expr, Class asType) throws OgnlException {
        Object value = null;
        try {
            expr = lookupForOverrides(expr);
            value = getValue(expr, asType);
            if (value == null) {
                value = findInContext(expr);
            }
        } finally {
            context.remove(THROW_EXCEPTION_ON_FAILURE);
        }
        return value;
    }

    private Object getValue(String expr, Class asType) throws OgnlException {
        return ognlUtil.getValue(expr, context, root, asType);
    }

    private Object findInContext(String name) {
        return getContext().get(name);
    }

    public Object findValue(String expr, Class asType) {
        return findValue(expr, asType, false);
    }

    /**
     * Log a failed lookup, being more verbose when devMode=true.
     *
     * @param expr The failed expression
     * @param e    The thrown exception.
     */
    private void logLookupFailure(String expr, Exception e) {
        String msg = LoggerUtils.format("Caught an exception while evaluating expression '#0' against value stack", expr);
        if (devMode && LOG.isWarnEnabled()) {
            LOG.warn(msg, e);
            LOG.warn("NOTE: Previous warning message was issued due to devMode set to true.");
        } else if (LOG.isDebugEnabled()) {
            LOG.debug(msg, e);
        }
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#peek()
     */
    public Object peek() {
        return root.peek();
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#pop()
     */
    public Object pop() {
        return root.pop();
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#push(java.lang.Object)
     */
    public void push(Object o) {
        root.push(o);
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#set(java.lang.String, java.lang.Object)
     */
    public void set(String key, Object o) {
        //set basically is backed by a Map pushed on the stack with a key being put on the map and the Object being the value
        Map setMap = retrieveSetMap();
        setMap.put(key, o);
    }

    private Map retrieveSetMap() {
        Map setMap;
        Object topObj = peek();
        if (shouldUseOldMap(topObj)) {
            setMap = (Map) topObj;
        } else {
            setMap = new HashMap();
            setMap.put(MAP_IDENTIFIER_KEY, "");
            push(setMap);
        }
        return setMap;
    }

    /**
     * check if this is a Map put on the stack  for setting if so just use the old map (reduces waste)
     */
    private boolean shouldUseOldMap(Object topObj) {
        return topObj instanceof Map && ((Map) topObj).get(MAP_IDENTIFIER_KEY) != null;
    }

    /**
     * @see com.opensymphony.xwork2.util.ValueStack#size()
     */
    public int size() {
        return root.size();
    }

    private Object readResolve() {
        // TODO: this should be done better
        ActionContext ac = ActionContext.getContext();
        Container cont = ac.getContainer();
        XWorkConverter xworkConverter = cont.getInstance(XWorkConverter.class);
        CompoundRootAccessor accessor = (CompoundRootAccessor) cont.getInstance(PropertyAccessor.class, CompoundRoot.class.getName());
        TextProvider prov = cont.getInstance(TextProvider.class, "system");
        boolean allow = "true".equals(cont.getInstance(String.class, "allowStaticMethodAccess"));
        OgnlValueStack aStack = new OgnlValueStack(xworkConverter, accessor, prov, allow);
        aStack.setOgnlUtil(cont.getInstance(OgnlUtil.class));
        aStack.setRoot(xworkConverter, accessor, this.root, allow);

        return aStack;
    }


    public void clearContextValues() {
        //this is an OGNL ValueStack so the context will be an OgnlContext
        //it would be better to make context of type OgnlContext
        ((OgnlContext) context).getValues().clear();
    }

    public void setAcceptProperties(Set<Pattern> acceptedProperties) {
        securityMemberAccess.setAcceptProperties(acceptedProperties);
    }

    public void setPropertiesJudge(PropertiesJudge judge) {
        securityMemberAccess.setPropertiesJudge(judge);
    }

    public void setExcludeProperties(Set<Pattern> excludeProperties) {
        securityMemberAccess.setExcludeProperties(excludeProperties);
    }

}

由這個源代碼的紅色部分，發現值棧中的數據分倆部分村儲：root（棧結構，CompoundRoot）和context（map形式,OgnlContext）：

ValueStack的生命週期

ValueStack的是保存着request的請求域中的，因此它的生命週期與request的生命週期是相同的。當Struts2接收到一個請求時，會迅速創建ActionContext、Action、ValueStack,然後將Action存放到ValueStack中，所以Action中的所有實例變量都是可以通過OGNL表達式訪問的。當請求來的時候，Action，ValueStack的生命週期開始。當請求結束的時候，生命週期結束。

ValueStack的傳送帶機制

Struts2的Action類通過屬性就可以獲得所有相關的值，如請求參數，action配置參數，向其他Action傳遞屬性值等。要獲得這些值，我們需要做的就是在Action類中聲明與參數同名的屬性，在Action執行默認的execute方法之前，Struts2就爲Action中的屬性進行了賦值。要完成這個功能很大程度上依賴於ValueStack對象。當 Struts 2接收到一個.action的請求後，會先建立Action類的對象實例，並且將Action類的對象實例壓入ValueStack對象中（實際 上，ValueStack對於相當一個棧），而ValueStack類的setValue和findValue方法可以設置和獲得Action對象的屬性 值。Struts 2中的某些攔截器正是通過ValueStack類的setValue方法來修改Action類的屬性值的。如params攔截器用於將請求參數值映射到相 應成Action類的屬性值。在params攔截器中在獲得請求參數值後，會使用setValue方法設置相應的Action類的屬性。從這一點可以看出，ValueStack對象就象一個傳送帶，當客戶端請求.action時，Struts 2在創建相應用Action對象後就將Action對象放到了ValueStack傳送帶上，然後ValueStack傳送帶會帶着Action對象經過 若干攔截器，在每一攔截器中都可以通過ValueStack對象設置和獲得Action對象中的屬性值。實際上，這些攔截器就相當於流水線作業。如果要對 Action對象進行某項加工，再加一個攔截器即可，當不需要進行這項工作時，直接將該攔截器去掉即可。