当return遇到finally

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当return遇到finally

sulong 于 2007-12-24

猜猜下面java程序分别会有什么样的输出？

程序:

public class Test {

  public static void main(String[] args) {
  System.out.println("=============test1==================");
  System.out.println(test1());
  System.out.println("===============================");

  System.out.println("=============test1_1==================");
  System.out.println(test1_1());
  System.out.println("===============================");

  System.out.println("\n============test2===================");
  System.out.println(test2());
  System.out.println("===============================");

  System.out.println("\n============test2_1===================");
  System.out.println(test2_1());
  System.out.println("===============================");

  System.out.println("\n============test3===================");
  System.out.println(test3());
  System.out.println("===============================");

  System.out.println("\n============test3_1===================");
  System.out.println(test3_1());
  System.out.println("===============================");
  }

  public static String test1() {
  String a = "in try";

  try{
  return a;
  } catch ( Exception e ) {

  } finally {
a = "in finally";
  System.out.println("do finally");
  }

  return a;
  }

  public static String test1_1() {
  String a = "in try";

  try{
  return a;
  } catch ( Exception e ) {

  } finally {
a = "in finally";
  System.out.println("do finally");
  return a;
  }
  }

  public static int test2() {
  int a = 1;

  try{
  return a;
  } catch ( Exception e ) {

  } finally {
a = 2;
  System.out.println("do finally");
  }

  return a;
  }

  public static int test2_1() {
  int a = 1;

  try{
  return a;
  } catch ( Exception e ) {

  } finally {
a = 2;
  System.out.println("do finally");
  return a;
  }
  }

  public static Helper test3() {
Helper a = new Helper();
a.a = 1;

  try{
  return a;
  } catch ( Exception e ) {

  } finally {
a.a = 2;
  System.out.println("do finally");
  }

  return a;
  }

  public static Helper test3_1() {
Helper a = new Helper();
a.a = 1;

  try{
  return a;
  } catch ( Exception e ) {

  } finally {
a.a = 2;
  System.out.println("do finally");
  return a;
  }
  }

  static class Helper {
  int a;

  public String toString() {
  return String.valueOf(a);
  }
  }
}

结果：

=============test1==================
do finally
in try
===============================
=============test1_1==================
do finally
in finally
===============================

============test2===================
do finally
1
===============================

============test2_1===================
do finally
2
===============================

============test3===================
do finally
2
===============================

============test3_1===================
do finally
2
===============================

结论：

在try catch块里return的时候，finally也会被执行。
return 语句会把后面的值复制到一份用来返回，如果return的是基本类型的，finally里对变量的改动将不起效果，如果return 的是引用类型的，改动将可以起效果。
finally里的return语句会把try catch块里的return语句效果给覆盖掉。

看来return语句并不一定都是函数的出口，执行return时，只是把return后面的值复制了一份到返回值变量里去了。看来最佳实践是：