惰性地实例化最后一个字段

不。最后一个字段的要点是,它在构建期间设置一次,此后将永远不会更改。编译器或虚拟机如何知道任何有用的信息 conn 对你来说?它怎么知道只有该属性才能设置它,而不能设置其他方法?

也许如果你解释一下你想要的语义是什么,我们可以想出一个替代词。您可能有一个“provider”接口,表示获取值的方法,然后是 MemoizingProvider

这里有一种方法可以使用备忘录(带可调用项)实现:

课程备忘录:

public class Memo<T> {
    private T result;
    private final Callable<T> callable;

    private boolean established;

    public Memo(final Callable<T> callable) {
        this.callable = callable;
    }

    public T get() {
        if (!established) {
            try {
                result = callable.call();
                established = true;
            }
            catch (Exception e) {
                throw new RuntimeException("Failed to get value of memo", e);
            }
        }
        return result;
    }
}

private final Memo<Connection> conn = new Memo<Connection>(
    new Callable<Connection>() {
    public Connection call() throws Exception {
        return new Connection();
    }
});

public Connection getConnection() {
    return conn.get();
}

Source

不要使用。

public class Test{
    private final Object mutex = new Object(); // No public locking
    private Connection conn;

    public Connection getConnection(){
        if(conn==null){
            synchronized (mutex) {
                if(conn==null){
                    conn = new Connection();
                }
            }
        }
        return conn;
    }
}

作为旁注,可以更改最后一个字段。至少包含实例字段。你只需要思考一下:

import java.lang.reflect.Field;

public class LazyFinalField {

  private final String finalField = null;   

  public static void main(String[] args) throws Exception {
    LazyFinalField o = new LazyFinalField();
    System.out.println("Original Value = " + o.finalField);
    Field finalField = LazyFinalField.class.getDeclaredField("finalField");
    finalField.setAccessible(true);
    finalField.set(o, "Hello World");   
    System.out.println("New Value = " + o.finalField);   
  }
}


Original Value = null
New Value = Hello World