Figure 103 Retrieving data from a database in Java

Make QR-Code in Java Figure 103 Retrieving data from a database
Figure 103 Retrieving data from a database
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Updates are shown in Figure 104 A client asks the mapper to save a domain object The mapper pulls the data out of the domain object and shuttles it to the database
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Figure 104 Updating data
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The whole layer of Data Mapper can be substituted, either for testing purposes or to allow a single domain layer to work with different databases
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A simple Data Mapper would just map a database table to an equivalent in-memory class on a field-to-field basis Of course, things aren't usually simple Mappers need a variety of strategies to handle classes that turn into multiple fields, classes that have multiple tables, classes with inheritance, and the joys of connecting together objects once they've been sorted out The various object-relational mapping patterns in this book are all about that It's usually easier to deploy these patterns with a Data Mapper than it is with the other organizing alternatives
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When it comes to inserts and updates, the database mapping layer needs to understand what objects have changed, which new ones have been created, and which ones have been destroyed It also has to fit the whole workload into a transactional framework The Unit of Work (184) pattern is a good way to organize this
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Figure 103 suggests that a single request to a find method results in a single SQL query This isn't always true Loading a typical order with multiple order lines may involve loading the order lines as well The request from the client will usually lead to a graph of objects being loaded, with the mapper designer deciding exactly how much to pull back in one go The point of this is to minimize database queries, so the finders typically need to know a fair bit about how clients use the objects in order to make the best choices for pulling data back
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This example leads to cases where you load multiple classes of domain objects from a single query If you want to load orders and order lines, it will usually be faster to do a single query that joins the orders and order line tables You then use the result set to load both the order and the order line instances (page 243)
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Since objects are very interconnected, you usually have to stop pulling the data back at some point Otherwise, you're likely to pull back the entire database with a request Again, mapping layers have techniques to deal with this while minimizing the impact on the in-memory objects, using Lazy Load (200) Hence, the inmemory objects can't be entirely ignorant of the mapping layer They may need to know about the finders and a few other mechanisms
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An application can have one Data Mapper or several If you're hardcoding your mappers, it's best to use one for each domain class or root of a domain hierarchy If you're using Metadata Mapping (306), you can get away with a single mapper class In the latter case the limiting problem is your find methods With a large application it can be too much to have a single mapper with lots of find methods, so it makes sense to split
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these methods up by each domain class or head of the domain hierarchy You get a lot of small finder classes, but it's easy for a developer to locate the finder she needs
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As with any database find behavior, the finders need to use an Identity Map (195) in order to maintain the identity of the objects read from the database Either you can have a Registry (480) of Identity Maps (195), or you can have each finder hold an Identity Map (195) (providing there is only one finder per class per session) Handling Finders
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In order to work with an object, you have to load it from the database Usually the presentation layer will initiate things by loading some initial objects Then control moves into the domain layer, at which point the code will mainly move from object to object using associations between them This will work effectively providing that the domain layer has all the objects it needs loaded into memory or that you use Lazy Load (200) to load in additional objects when needed
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On occasion you may need the domain objects to invoke find methods on the Data Mapper However, I've found that with a good Lazy Load (200) you can completely avoid this For simpler applications, though, may not be worth trying to manage everything with associations and Lazy Load (200) Still, you don't want to add a dependency from your domain objects to your Data Mapper
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You can solve this dilemma by using Separated Interface (476) Put any find methods needed by the domain code into an interface class that you can place in the domain package Mapping Data to Domain Fields
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Mappers need access to the fields in the domain objects Often this can be a problem because you need public methods to support the mappers you don't want for domain logic (I'm assuming that you won't commit the cardinal sin of making fields public) There's no easy to answer to this You could use a lower level of visibility by packaging the mappers closer to the domain objects, such as in the same package in Java, but this confuses the bigger dependency picture because you don't want other parts of the system that know the domain objects to know about the mappers You can use reflection, which can often bypass the visibility rules of the language It's slower, but the slower speed may end up as just a rounding error compared to the time taken by the SQL call Or you can use public methods, but guard them with a status field so that they throw an exception if they're used outside the context of a database load If so, name them in such a way that they're not mistaken for regular getters and setters
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Tied to this is the issue of when you create the object In essence you have two options One is to create the object with a rich constructor so that it's at least created with all its mandatory data The other is to create an empty object and then populate it with the mandatory data I usually prefer the former since it's nice to have a well-formed object from the start This also means that, if you have an immutable field, you can enforce it by not providing any method to change its value
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The problem with a rich constructor is that you have to be aware of cyclic references If you have two objects that reference each other, each time you try to load one it will try to load the other, which will in turn try to load the first one, and so on, until you run out of stack space Avoiding this requires special case code, often using Lazy Load (200) Writing this special case code is messy, so it's worth trying to do without it You can do this by creating an empty object Use a no-arg constructor to create a blank object and insert that empty object immediately into the Identity Map (195) That way, if you have a cycle, the Identity Map (195) will return an object to stop the recursive loading
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Using an empty object like this means you may need some setters for values that are truly immutable when the object is loaded A combination of a naming convention and perhaps some status-checking guards can fix this You can also use reflection for data loading Metadata-Based Mappings
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One of the decisions you need to make concerns storing the information about how fields in domain objects are mapped to columns in the database The simplest, and often best, way to do this is with explicit code, which requires a mapper class for each domain object The mapper does the mapping through assignments and has fields (usually constant strings) to store the SQL for database access An alternative is to use Metadata Mapping (306), which stores the metadata as data, either in a class or in a separate file The great advantage of metadata is that all the variation in the mappers can be handled through data without the need for more source code, either by use of code generation or reflective programming
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