mybatis plugin

Before we know that the private void parseConfiguration(XNode root) method is called during the configuration creation process, there is a pluginElement(root.evalNode("plugins")); method, this is the plugin that reads the configuration;

  private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        String interceptor = child.getStringAttribute("interceptor");
        Properties properties = child.getChildrenAsProperties();
        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
        interceptorInstance.setProperties(properties);
        configuration.addInterceptor(interceptorInstance);
      }
    }
  }

See this sentence:

Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();

Yes, this is the plug-in that reads the plug-in node and configuration parameters during the initialization of the mybatis context, and generates a plug-in instance through reflection technology;

configuration.addInterceptor(interceptorInstance);

Put the plugin into the configuration and follow up:

public class InterceptorChain {

  private final List<Interceptor> interceptors = new ArrayList<Interceptor>();

  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }
  
  public List<Interceptor> getInterceptors() {
    return Collections.unmodifiableList(interceptors);
  }

}

Finally, the plugin is saved in a list object and waiting to be called.

There is a plug-in interface that needs to be said, that is Interceptor, enter it:

public interface Interceptor {

  Object intercept(Invocation invocation) throws Throwable;

  Object plugin(Object target);

  void setProperties(Properties properties);

}

There are only three methods in this interface, so let's introduce the function of each method:

1) The intercept() method is the core method of the plug-in, which directly overwrites the original method of the intercepted object;

2) The plugin() method is to generate a proxy object for the intercepted object and return it; to explain more here, mybatis provides the wrap static method in org.apache.ibatis.plugin.Plugin to generate a proxy object;

3) setProperties() allows all parameters to be configured in the plugin element;

We will call when we get the four directions

  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

The plugin() here is the method 2) in the interface, the purpose is to generate a proxy object;