Groovy 中的 with

Java与Groovy闭包技术对比

最新推荐文章于 2025-08-20 12:37:37 发布

原创最新推荐文章于 2025-08-20 12:37:37 发布 · 1.2k 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#java

Groovy 专栏收录该内容

7 篇文章

订阅专栏

本文通过对比Java和Groovy中闭包的应用，详细介绍了如何使用Java的PrintIndependenceDay示例来实现日期打印功能，并展示了Groovy中闭包的特性与使用方法。通过代码示例和解析，深入探讨了闭包在Java和Groovy中的不同实现方式及其实现逻辑。

in Java

// PrintIndependenceDay.java

import java.util.Calendar;
import java.util.Date;

public class PrintIndependenceDay {

  public static void main(String[] args) {
    Calendar calendar = Calendar.getInstance();
    calendar.clear();
    calendar.set(Calendar.MONTH, Calendar.JULY);
    calendar.set(Calendar.DATE, 4);
    calendar.set(Calendar.YEAR, 1776);
    Date time = calendar.getTime();
    System.out.println(time);
  }
}

in Groovy:

// PrintIndependenceDay.groovy

def calendar = Calendar.instance
calendar.with {
  clear()
  set MONTH, JULY
  set DATE, 4
  set YEAR, 1776
  println time
}

每个 Groovy闭包有个与其关联的delegate(代理人).

// define a closure
def myClosure = {
  // call a method that does not exist
  append 'Jeff'
  append ' was here.'
}

// assign a delegate to the closure
def sb = new StringBuffer()
myClosure.delegate = sb

// execute the closure
myClosure()

assert 'Jeff was here.' == sb.toString()

换一种写法:

def closure = {
  clear()
  set MONTH, JULY
  set DATE, 4
  set YEAR, 1776
  println time
}
def calendar = Calendar.instance
closure.delegate = calendar
closure()

Another bit of info that is missing here is the strategy that a closure uses to decide when to send method calls to the delegate. Each Groovy closure has a resolveStrategy associated with it. This property determines how/if the delegate comes in to play. The 4 possible values for the resolveStrategy are OWNER_FIRST, DELEGATE_FIRST, OWNER_ONLY and DELEGATE_ONLY (all constants defined in groovy.lang.Closure). The default is OWNER_FIRST. Consider the owner to be the "this" wherever the closure is defined. Here is a simple example...

class ResolutionTest {
    
    def append(arg) {
        println "you called the append method and passed ${arg}"
    }
    
    def doIt() {
        def closure = {
            append 'Jeff was here.'
        }
        
        def buffer = new StringBuffer()
        
        closure.delegate = buffer
        
        // the append method in this ResolutionTest
        // will be called because resolveStrategy is
        // OWNER_FIRST (the default)
        closure()
        
        // give the delegate first crack at method
        // calls made inside the closure
        closure.resolveStrategy = Closure.DELEGATE_FIRST
        
        // the append method on buffer will
        // be called because the delegate gets
        // first crack at the call to append()
        closure()
    }
    
    static void main(String[] a) {
        new ResolutionTest().doIt()
    }
}

运行结果:

you called the append method and passed Jeff was here.

来源:

http://java.dzone.com/news/getting-groovy-with-with