The criteria API is a way to programmatically create dynamic JPA queries in a type safe manner. It is supposed to be used when the JPA query cannot be statically defined up front because the actual query depends on input provided by the user - e.g. a search form with many optional filter conditions that can be flexibly combined in any possible way.
In hibernate the criteria API is used to create a graph of hibernate objects (implementations of the org.hibernate.Criteria interface and related objects) representing the required query. This graph of objects is then internally transformed to a JPQL query (basically a string). This string is then parsed and mapped to a SQL query. It is interesting to notice, that hibernate does not map the criteria object graph directly to SQL but rather goes through an intermediate JPQL query string. This process is repeated every time a criteria API query is executed.
Obviously, this process requires a lot of CPU cycles each time a query is executed. In cases when such queries are run very often this might already be a prohibitive factor. Unfortunately, it gets even worse.
The even more concerning issue is part of the process of transforming JPQL queries to SQL. Hibernate tries to load some of the tokens in the JPQL query as classes to determine whether they are actually classes. This results in hibernate calling ClassLoader.loadClass() with parameters like "a0", "b1" etc.
Class-loaders are organized in hierarchies. In an enterprise application a WAR class-loader would have a parent EAR class-loader with a parent the application server class-loader and so on. The default behaviour of each class-loader, when looking for a class, is to first try to load the class from the parent class-loader. If this fails, it will try to load the class itself. As a result of this strategy, trying to load a non-existent class is pretty much the worst-case scenario for a class-loader - it has to look for the class in the root class-loader and in each class-loader down the hierarchy to finally determine that the class is not known and throw a ClassNotFoundException. This has two issues. First of all, normally class-loaders do not cache such cases. When a class is found, some class-loaders might remember that (cache it) so that subsequent calls are much faster. Missing classes are usually not cached. This means that looking for a non-existent class (even the same one) over and over again always hits this worst-case scenario causing each class-loader to go through all of its classes. The second issue lays in the way the some class-loaders (e.g. the WebLogic application server class-loader) which assume that loading classes does not happen that often and use a very coarse-grained locking - loading a class basically locks the class-loader through the whole search process.
Lets get back to our criteria API query. One enterprise application obviously has one single class-loader instance (and the same class-loaders on the way up in the hierarchy). Each criteria API query tries to get the lock on the same class-loader while looking for JPQL tokens. This results in all criteria API queries contending for the same locks - a high lock contention in the class-loader and sync times are what follows.
In this blog post we saw that one needs to be willing to pay a very high price to use use criteria API with hibernate. Depending on the performance requirements it might be okay for some applications. For others, however, that is absolutely prohibitive and any high-load process must not use criteria API. My advice as a performance engineer is to avoid criteria APIs as much as possible. In many cases it is possible to use a few named queries (which, by the way, go through the same JPQL to SQL process but only once upon start-up) or even revert to native SQL or other technologies.