ivaneeo's blog

First you have to create a freemarker.template.Configuration instance and adjust its settings. A Configuration instance is a central place to store the application level settings of FreeMarker. Also, it deals with the creation and caching of pre-parsed templates.

Probably you will do it only once at the beginning of the application (possibly servlet) life-cycle:

二. 具有不同配置（set....）的 Configuration 應該建立相互獨立的實例：

From now you should use this single configuration instance. Note however that if a system has multiple independent components that use FreeMarker, then of course they will use their own private Configuration instance.

三. 共享的 Configuration 實例有利于開啟 MRU Cache 功能：

In a multithreaded environment Configuration instances, Template instances and data models should be handled as immutable (read-only) objects. That is, you create and initialize them (for example with set... methods), and then you don't modify them later (e.g. you don't call set...). This allows us to avoid expensive synchronized blocks in a multithreaded environment. Beware with Template instances; when you get a Template instance with Configuration.getTemplate, you may get an instance from the template cache that is already used by other threads, so do not call its set... methods (calling process is of course fine).

The above restrictions do not apply if you access all objects from the same single thread only.

四. 開啟 MRU Cache 策略

FreeMarker caches templates (assuming you use the Configuration methods to create Template objects). This means that when you call getTemplate, FreeMarker not only returns the resulting Template object, but stores it in a cache, so when next time you call getTemplate with the same (or equivalent) path, it just returns the cached Template instance, and will not load and parse the template file again.

cfg.setCacheStorage(new freemarker.cache.MruCacheStorage(20, 250))

Or, since MruCacheStorage is the default cache storage implementation:

	cfg.setSetting(Configuration.CACHE_STORAGE_KEY, "strong:20, soft:250");

When you create a new Configuration object, initially it uses an MruCacheStorage where maxStrongSize is 0, and maxSoftSize is Integer.MAX_VALUE (that is, in practice, infinite). But using non-0 maxStrongSize is maybe a better strategy for high load servers, since it seems that, with only softly referenced items, JVM tends to cause just higher resource consumption if the resource consumption was already high, because it constantly throws frequently used templates from the cache, which then have to be re-loaded and and re-parsed.

五. MRU （Most Recently Used） Cache 自動更新模板內容的特性

If you change the template file, then FreeMarker will re-load and re-parse the template automatically when you get the template next time. However, since checking if the file has been changed can be time consuming, there is a Configuration level setting called ``update delay''. This is the time that must elapse since the last checking for a newer version of a certain template before FreeMarker will check that again. This is set to 5 seconds by default. If you want to see the changes of templates immediately, set it to 0. Note that some template loaders may have problems with template updating. For example, class-loader based template loaders typically do not notice that you have changed the template file.

六. MRU Cache 的兩級緩存策略

A template will be removed from the cache if you call getTemplate and FreeMarker realizes that the template file has been removed meanwhile. Also, if the JVM thinks that it begins to run out of memory, by default it can arbitrarily drop templates from the cache. Furthermore, you can empty the cache manually with the clearTemplateCache method of Configuration.

The actual strategy of when a cached template should be thrown away is pluggable with the cache_storage setting, by which you can plug any CacheStorage implementation. For most users freemarker.cache.MruCacheStorage will be sufficient. This cache storage implements a two-level Most Recently Used cache. In the first level, items are strongly referenced up to the specified maximum (strongly referenced items can't be dropped by the JVM, as opposed to softly referenced items). When the maximum is exceeded, the least recently used item is moved into the second level cache, where they are softly referenced, up to another specified maximum. The size of the strong and soft parts can be specified with the constructor. For example, set the size of the strong part to 20, and the size of soft part to 250: