如何在freemarker中检查两个java对象是否相同

Question

我需要比较 freemarker 中的两个 java 对象是否相同。有没有办法检查freemarker中两个java对象的相等性

Answer 1

不幸的是，从 2.3.19 开始就没有这种可能性了；可能会在 2.3.20 左右。你必须写一个TemplateMethodModelEx 来实现这个，比如sameObjects(p1, p2)。在这种情况下，您必须从参数TemplateModel-s 中提取原始对象。为此，您必须检查参数是否实现AdapterTemplateModel，然后调用AdapterTemplateModel.getAdaptedObject(Object.class)。然后您可以将原始对象与== 进行比较，仍然是Java，然后返回true/false。

更新：由于我打算将其贡献给 FreeMarker，因此我进行了更多研究。使用AdapterTemplateModel 对此并不完全正确，因为它可能涉及转换（如 Python 到 Java），然后您会丢失原始对象的身份，从而得到假阴性。使用WrapperTemplateModel 看起来像解决方案，但事实证明它对 Jython 的实现是错误的......所以我看到的唯一解决方案永远不会给出不正确的结果（但可能会给出错误，因为无法进行比较）是BeanModel。具体实现如下：

package com.example;

import java.util.List;

import freemarker.ext.beans.BeanModel;
import freemarker.ext.beans.BeansWrapper;
import freemarker.template.TemplateBooleanModel;
import freemarker.template.TemplateMethodModelEx;
import freemarker.template.TemplateModelException;

/**
 * Checks if two values correspond to the same object. This only works if both
 * arguments are wrapped into {@link BeanModel}-s by the object wrapping
 * facility of FreeMarker, which is usually the case for objects that aren't
 * {@code Collection}-s, {@code Map}-s, {@code String}-s, {@code Number}-s,
 * {@code Date-s}, {@code Boolean}-s, Jython objects, Rhino objects or DOM
 * objects. If you are using pure {@link BeansWrapper} for wrapping, this is the
 * case for all objects. If not all the arguments are {@link BeanModel}-s, or
 * some of them are {@code null}-s, this will throw an exception.
 */
public class IsSameObjectMethodModel implements TemplateMethodModelEx {

    public Object exec(List args) throws TemplateModelException {
        if (args.size() != 2) {
            throw new TemplateModelException(
                    "Method expects exactly 2 arguments, but " +
                    args.size() + " was given.");
        }
        return toRawArg("1st", args.get(0)) == toRawArg("2nd", args.get(1)) ?
                TemplateBooleanModel.TRUE : TemplateBooleanModel.FALSE;
    }

    /**
     * Extracts the original object from the argument.
     * @param argName Used in error messages
     */
    private Object toRawArg(String argName, Object argVal)
    throws TemplateModelException {
        if (argVal == null) throw new TemplateModelException(
                "Method doesn't support null arguments, but the " +
                argName + " argument was null");
        if (argVal instanceof BeanModel) {
            return ((BeanModel) argVal).getWrappedObject(); 
        } else {
            throw new TemplateModelException(
                    "Method only supports arguments that were wrapped by " +
                    "FreeMarker (or something else) so that they extend " +
                    "freemarker.ext.beans.BeanModel, but " +
                    "the " + argName + " argument wasn't like that (class: " +
                    argVal.getClass().getName() + "). To avoid this error, " +
                    "avoid comparing objects that are Collection-s, " +
                    "Map-s, String-s, Number-s, Date-s, Boolean-s, Jython " +
                    "objects, Rhino objects or DOM objects.");
        }
    }

}

要使用它，请将其放入数据模型中，或者假设您有一些用于导入/包含的模板，比如说utils.ftl：

...
[#assign isSameObject = "com.example.IsSameObjectMethodModel"?new()]
...

然后在模板中：

[#import "utils.ftl" as u]
...
[#if u.isSameObject(o1, o2)]
  same
[#else]
  different
[/#if]