JAVA try-catch-finally-return

 
  • 正常执行流程
try执行,遇到异常就跳到catch执行(以使得程序不会崩溃);
不管有没有异常catch,最后都执行finally
 
  • 含return语句执行流程分析
JAVA try-catch-finally-return
若try块中return 可达, return语句(如:return x=x+1;)
  1. 对x执行运算x=x+1 (若有运算)
  2. 复制一个变量x给try的return语句(按值复制:基本类型就是值本身,对象就是地址值)
  3. 但return语句并不马上返回,控制权转移到finally块中执行: (1)若finally中无return语句:finally执行完后,try块执行return语句返回之前复制的变量x(基本类型就是值本身,对象就是地址值)(所以:若x为基本类型,fianlly中改变x对最终return结果无效;若x为对象类型,按地址值传递可以改变x的最终return结果)    (2)若finally中有return语句:执行后直接返回(即“覆盖try中return语句”:try中return语句不再返回)
 
若try中return 不可达(exception 在return前出现
    exception出现后控制权直接转到catch 块(即try中 exception之后的语句<包括return>不会执行),最后到finally块(catch到finally中流程与上面try到finally中流程相同)
 
 
  • finally不执行的特殊情况
  1. if you call System.exit() or
  2. if the JVM crashes first
JAVA try-catch-finally-return
 
  • A return statement in the finally block is a bad idea
By doing a return from the finally block, you suppress the exception entirely.(finally 中有return ,try,catch的 return throw都不会再被调用)
  1. publicstaticint getANumber(){
  2.     try{
  3.         thrownewNoSuchFieldException();
  4.     } finally {
  5.         return43;
  6.     }
  7. }
Running the method above will return a “43” and the exception in the try block will not be thrown. This is why it is considered to be a very bad idea to have a return statement inside the finally block.
 
 

例子解释:
 
执行流程
JAVA try-catch-finally-return
  1. If the return in the try block is reached, it transfers control to the finally block, and the function eventually returns normally (not a throw).
  2. If an exception occurs, but then the code reaches a return from the catch block, control is transferred to the finally block and the function eventually returns normally (not a throw).
  3. In your example, you have a return in the finally, and so regardless of what happens, the function will return 34, because finally has the final (if you will) word.
 
 
finally语句在try或catch中的return语句执行之后、return返回之前执行的
  • 在try中含有return+基本类型情况:
  1. publicclassFinallyTest1{
  2.     publicstaticvoid main(String[] args){
  3.         System.out.println(test1());
  4.     }
  5.     publicstaticint test1(){
  6.         int b =20;
  7.         try{
  8.             System.out.println("try block");
  9.             return b +=80;
  10.         }
  11.         catch(Exception e){
  12.             System.out.println("catch block");
  13.         }
  14.         finally {
  15.             System.out.println("finally block");
  16.             if(b >25){
  17.                 System.out.println("b>25, b = "+ b);
  18.             }
  19.         }
  20.         return b;
  21.     }
  22. }
运行结果:
  1. try block
  2. finally block
  3. b>25, b =100
  4. 100
 
  • 在catch中含有return+基本类型情况(分析跟在try中含有return情况一样):
  1. publicclassTest{
  2.  
  3.     publicstaticvoid main(String[] args){
  4.  
  5.         System.out.println(test1());
  6.     }
  7.  
  8.     publicstaticint test1(){
  9.         int b =20;
  10.  
  11.         try{
  12.             int a =1/0;//触发异常
  13.             System.out.println("try block");//不会被执行
  14.             return b +=80;//不会被执行
  15.         }
  16.         catch(Exception e){
  17.             System.out.println("catch block");
  18.             return b +=180;//最后在此返回
  19.         }
  20.         finally {
  21.             System.out.println("finally block");
  22.             if(b >25){
  23.                 System.out.println("b>25, b = "+ b);
  24.             }
  25.         }
  26. //        return b;
  27.     }
  28.  
  29. }
执行结果:
  1. catch block
  2. finally block
  3. b>25, b =200
  4. 200
 
  • 在try中含有return+对象类型情况:
  1. publicclassTest{
  2.  
  3.     publicstaticvoid main(String[] args){
  4.         System.out.println(getMap().get("KEY").toString());
  5.     }
  6.  
  7.     publicstaticMap<String,String> getMap(){
  8.         Map<String,String>map=newHashMap<String,String>();
  9.         map.put("KEY","INIT");
  10.  
  11.         try{
  12.             map.put("KEY","TRY");
  13.             returnmap;//return在控制权转移到finally前:复制了一个map对象的地址值(对象按地址值传递)
  14.         }
  15.         catch(Exception e){
  16.             map.put("KEY","CATCH");
  17.         }
  18.         finally {
  19.             map.put("KEY","FINALLY");//根据对象地址值修改对象内容(按地址值传递),因此会影响try中return返回的对象内容
  20.             map= null;//map为null即不再指向该对象,
  21.             // 但由于前面return复制了一个对象的引用(地址值),而对象是被分配在堆中的,只要有引用指向这个对象,系统就不会回收此对象, 所以此处map = null 并不会影响最后try中return返回对象内容
  22.         }
  23.  
  24.         returnmap;
  25.     }
  26.  
  27. }
执行结果:
  1. FINALLY
  • 在catch中含有return+基本类型情况(分析跟在try中含有return情况一样):
  1. publicclassTest{
  2.  
  3.     publicstaticvoid main(String[] args){
  4.         System.out.println(getMap().get("KEY").toString());
  5.     }
  6.  
  7.     publicstaticMap<String,String> getMap(){
  8.         Map<String,String>map=newHashMap<String,String>();
  9.         map.put("KEY","INIT");
  10.  
  11.         try{
  12.             int a =1/0;//触发异常
  13.             map.put("KEY","TRY");//不会被执行
  14.             returnmap;//不会被执行
  15.         }
  16.         catch(Exception e){
  17.             map.put("KEY","CATCH");
  18.             returnmap;//return在控制权转移到finally前:复制了一个map对象的地址值(对象按地址值传递)
  19.         }
  20.         finally {
  21.             map.put("KEY","FINALLY");//根据对象地址值修改对象内容(按地址值传递),因此会影响catch中return返回的对象内容
  22.             map= null;//map为null即不再指向该对象,
  23.             // 但由于前面return复制了一个对象的引用(地址值),而对象是被分配在堆中的,只要有引用指向这个对象,系统就不会回收此对象,
  24.             // 所以此处map = null 并不会影响最后catch中return返回对象内容
  25.         }
  26.  
  27. //        return map;
  28.     }
  29.  
  30. }
执行结果:
  1. FINALLY
 
 
finally块中的return语句会覆盖try块中的return返回
  1. publicclassTest{
  2.  
  3.     publicstaticvoid main(String[] args){
  4.  
  5.         System.out.println(test2());
  6.     }
  7.  
  8.     publicstaticint test2(){
  9.         int b =20;
  10.  
  11.         try{
  12.             System.out.println("try block");
  13.             return b +=80;
  14.         }catch(Exception e){
  15.             System.out.println("catch block");
  16.         } finally {
  17.             System.out.println("finally block");
  18.             if(b >25){
  19.                 System.out.println("b>25, b = "+ b);
  20.             }
  21.             return200;
  22.         }
  23.         // return b;
  24.     }
  25.  
  26. }
执行结果:
  1. try block
  2. finally block
  3. b>25, b =100
  4. 200
 
参考:http://www.cnblogs.com/lanxuezaipiao/p/3440471.html(博文代码用例4结果有误,见该博文讨论区分析)
 
 
 
 
 
 
 
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