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1. How does dependency injection improve scalability

Extensibility is improved when an interface is injected and a specific interface implementation class is injected by the caller.

2. Timing of dependency injection

In Spring, dependency injection works in the following ways:

  1. Through class member injection
  2. Injection through the constructor
  3. Injection through method parameters

Ordinary Java programs can also be injected in these ways, so when and which dependency injection methods are appropriate? This should be considered from the following aspects:

scalability

With property injection, the injected class is already written dead at the time of class definition and cannot be changed. Constructor injection and method injection, on the other hand, can be passed in by the caller at invocation time, increasing extensibility.

The life cycle

Property injection and constructor injection mean that the injected class’s life cycle is the same as that of the host class. The injected class exists as long as the host class exists. By method injection, however, it means that the injected class is normally used only within the method. When the method ends, the injected class’s life cycle ends, freeing up memory.

Based on these two points, it is easy to determine when and which injection method should be used.

3. Examples of dependency injection to improve scalability

Let’s look at how dependency injection can improve extensibility through the ThreadPoolExecutor class in the Java JUC package.


class Duties and responsibilities
ThreadPoolExecutor Thread executor
BlockingQueue The buffer queue used by the thread pool. The queue length determines the maximum number of worker threads that can be buffered
Runnable Runnable in thread queue
ThreadFactory Create thread factories for worker threads
RejectedExecutionHandler Process policy interface for adding new threads when the thread queue in thread executor is full
DiscardOldestPolicy Discard the oldest pending worker thread in the thread queue and add a new worker thread.
CallerRunsPolicy Repeat until successful and use in closeConnectionExecutor.
AbortPolicy Discard worker threads that exceed thread queue load and throw an exception.
DiscardPolicy Worker threads that are complex beyond the thread queue are ignored and no processing is done.

The injection mode of each interface is constructor injection:

    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
Copy the code

4. To summarize

  1. Dependency injection is the best way to increase extensibility by leaving it to the caller with more freedom, rather than providing extensibility through multiple default implementations, which are not comparable in scale.
  2. Depending on the scenario, it is reasonable to determine whether the dependency injection method is constructor injection or method injection.
  3. Don’t use dependency injection all the time just because it provides great extensibility. Use it only when you need it, as applicable.

end.

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