freemaker 空值

The FreeMarker template language doesn't know the Java language null at all. It doesn't have null keyword, and it can't test if something is null or not. When it technically faces with a null, it treats it exactly as a missing variable. For example, both if x is null in the data-model and if it's not present at all, ${x!'missing'} will print ``missing'', you can't tell the difference. Also, if for example you want to test if a Java method has returned null, just write something like <#if foo.bar()??>.

You may interested in the rationale behind this. From the viewpoint of the presentation layer a null and non-existent thing is almost always the same. The difference between this two is usually just a technical detail, which is rather the result of implementation details than of the application logic. That you can't compare something to null (unlike in Java); it doesn't make sense to compare something with null in a template, since the template language doesn't do identity comparison (like the Java == operator when you compare two objects) but the more common sense value comparison (like Java's Object.equals(Object); that doesn't work with null either). And how could FreeMarker tell if something concrete equals with something that is missing and thus unknown? Or if two missing (unknown) things are equal? Of course these questions can't be answered.

There is at least one problem with this null-unaware approach. When you call a Java method from a template, you may want to pass a null value as argument (since the method was designed to be used in Java language, where the concept of null is known). In this case you can exploit a bug of FreeMarker (that we will not fix until we provide a correct solution for passing null values to a method): if you specify a missing variable as the argument, then it will not cause an error, but a null will be passed to the method instead. Like foo.bar(nullArg) will call the bar method with null as argument, assuming that there is no varaible exists with ``nullArg'' name.