return语句在Java的Try Catch Finally中的行为

来源：互联网发布：python redis py 编辑：程序博客网时间：2024/05/18 16:39

总结：
1.不管有无异常，finally中的代码都会执行

2.无论在try中返回,还是在catch中返回，都要先执行finally

3.如果finally中有return语句执行，那么try和catch中的return语句都会失效。程序在try或catch中return后会进入finally并最终会使用finally中的return进行返回

4.返回的结果在try或catch中return时，便已经准备好了，并存放到一个临时变量中，执行完finally后再从中取出，所以在finally中对原变量的修改不会影响返回值，基本数据类型和引用类型都满足

5.finally中不能改变返回的引用变量，因为有临时变量存在，改变无效。但是可以改变返回的引用变量所指的对象的内容（因为持有对象的引用，所以可以访问并修改）

下面是代码测试

package Blue2Te.Test;/** * Created by Blue2Te on 16/11/13. */public class TryFinallyTest {    /**     * Test测试     * 0无异常测试     * 1有异常测试     * 正常返回0     * try中返回为1     * catch中返回为2     * finally中返回为3     *     * specialTest测试     */    public static void main(String args[]){        //Test        /**         * Test1测试 try{}catch(){}finally{} return;         */        System.out.println(Test1(0));        System.out.println(Test1(1));        /**         * Test2 0测试 try{return} catch(){} finally{} return;         * Test2 1测试 try{} catch(){return} finally{} return;         */        System.out.println(Test2(0));        System.out.println(Test2(1));        /**         * Test3 0测试 try{return} catch(){} finally{return} ;         * Test3 1测试 try{} catch(){return} finally{return} ;         */        System.out.println(Test3(0));        System.out.println(Test3(1));        //specialTest        System.out.println("specialTes1 return: "+specialTest1());        System.out.println("specialTest1AddForCatch return: "+specialTest1AddForCatch());        MyPoint raw=new MyPoint();        MyPoint ret = specialTest2(raw);        System.out.println("specialTest2 comparation raw==ret: "+(raw == ret));        raw=new MyPoint();        ret = specialTest2AddForCatch(raw);        System.out.println("specialTest2AddForCatch comparation raw==ret: "+(raw == ret));        raw = new MyPoint();        System.out.println("Before specialTest3");        System.out.println(raw);        specialTest3(raw);        System.out.println("After specialTest3");        System.out.println(raw);        raw = new MyPoint();        System.out.println("Before specialTest3AddForCatch");        System.out.println(raw);        specialTest3AddForCatch(raw);        System.out.println("After specialTest3AddForCatch");        System.out.println(raw);    }    //Test    /*     不管有没有出现异常，finally块中代码都会执行     Test1 iCase0 try     Test1 iCase0 finally     0     Test1 iCase1 try     Test1 iCase1 catch     Test1 iCase1 finally     0     */    public static int Test1(int iCase){        if(iCase==0){            try {                System.out.println("Test1 iCase"+iCase+" try");            }catch (Exception e){                System.out.println("Test1 iCase"+iCase+" catch");            }finally {                System.out.println("Test1 iCase"+iCase+" finally");            }        }else{            try {                System.out.println("Test1 iCase"+iCase+" try");                throw new Exception("For Test1 Exception");            }catch (Exception e){                System.out.println("Test1 iCase"+iCase+" catch");            }finally {                System.out.println("Test1 iCase"+iCase+" finally");            }        }        return 0;    }    /*    无论在try中返回,还是在catch中返回，都要先执行finally    Test2 iCase0 try    Test2 iCase0 finally    1    Test2 iCase1 try    Test2 iCase1 catch    Test2 iCase1 finally    2     */    public static int Test2(int iCase){        if(iCase==0){            try {                System.out.println("Test2 iCase"+iCase+" try");                return iCase+1;            }catch (Exception e){                System.out.println("Test2 iCase"+iCase+" catch");            }finally {                System.out.println("Test2 iCase"+iCase+" finally");            }        }else{            try {                System.out.println("Test2 iCase"+iCase+" try");                throw new Exception("For Test2 Exception");            }catch (Exception e){                System.out.println("Test2 iCase"+iCase+" catch");                return iCase+1;            }finally {                System.out.println("Test2 iCase"+iCase+" finally");            }        }        return 0;    }    /*    如果finally中有return语句执行，那么try和catch中的return语句都会失效    程序在try或catch中return后会进入finally并最终会使用finally中的return进行返回    Test3 iCase0 try    Test3 iCase0 finally    3    Test3 iCase1 try    Test3 iCase1 catch    Test3 iCase1 finally    3     */    public static int Test3(int iCase){        if(iCase==0){            try {                System.out.println("Test3 iCase"+iCase+" try");                return iCase+1;            }catch (Exception e){                System.out.println("Test3 iCase"+iCase+" catch");            }finally {                System.out.println("Test3 iCase"+iCase+" finally");                return iCase+3;            }        }else{            try {                System.out.println("Test3 iCase"+iCase+" try");                throw new Exception("For Test3 Exception");            }catch (Exception e){                System.out.println("Test3 iCase"+iCase+" catch");                return iCase+1;            }finally {                System.out.println("Test3 iCase"+iCase+" finally");                return iCase+2;            }        }    }    //specialTest    /*    基本数据类型的测试    try var: 1    finally var: 2    specialTes1 return: 1    try var: 1    catch var: 2    finally var: 3    specialTest1AddForCatch return: 2    从结果可以看出，finally中对var的修改并没有影响到返回值    返回的结果在try或catch中return时，便已经准备好了，并存放到一个临时变量中，执行完finally后再从中取出    所以在finally中对原变量的修改不会影响返回值     */    public static int specialTest1(){        int var = 0;        try{            ++var;      //var=1;            System.out.println("try var: "+var);            return var;        }catch (Exception e){            e.printStackTrace();        }finally {            ++var;      //var=2;            System.out.println("finally var: "+var);        }        return -1;    }    public static int specialTest1AddForCatch(){        int var = 0;        try{            ++var;      //var=1;            System.out.println("try var: "+var);            throw new Exception();        }catch (Exception e){            ++var;      //var=2;            System.out.println("catch var: "+var);            return var;        }finally {            ++var;      //var=3;            System.out.println("finally var: "+var);        }    }    /*    引用类型的测试     */    static class MyPoint{        int x;        int y;        public MyPoint(){            x=0;            y=0;        }        @Override        public String toString() {            return "x: "+x+"  y: "+y;        }    }    /*    是否为同一个对象的测试    finally raw==newMyPoint:false    specialTest2 comparation raw==ret: true    finally raw==newMyPoint:false    specialTest2AddForCatch comparation raw==ret: true    结果说明引用变量的返回与基本数据类型一致，在return时会先将引用变量保存到一个临时变量中，再转去finally中执行，    finally中对引用变量的修改不影响前面的临时变量，finally结束后会将前面的临时变量返回。     */    public static MyPoint specialTest2(MyPoint raw){        MyPoint newMyPoint = new MyPoint();        try{            return raw;        }catch (Exception e){            e.printStackTrace();        }finally {            System.out.println("finally raw==newMyPoint:"+(raw==newMyPoint));            raw = newMyPoint;        }        return null;    }    public static MyPoint specialTest2AddForCatch(MyPoint raw){        MyPoint newMyPoint = new MyPoint();        try{            throw new Exception();        }catch (Exception e){            return raw;        }finally {            System.out.println("finally raw==newMyPoint:"+(raw==newMyPoint));            raw = newMyPoint;        }    }    /*    finally中不能改变返回的引用变量地址信息（specialTest2测试），因为有临时变量存在，改变无效    但是可以改变返回的引用变量所指的对象的内容（因为持有对象的引用，所以可以访问并修改）    Before specialTest3    x: 0  y: 0    After specialTest3    x: -1  y: -1    Before specialTest3AddForCatch    x: 0  y: 0    After specialTest3AddForCatch    x: -2  y: -2     */    public static MyPoint specialTest3(MyPoint raw){        try{            return raw;        }catch (Exception e){            e.printStackTrace();        }finally {            raw.x=-1;            raw.y=-1;        }        return null;    }    public static MyPoint specialTest3AddForCatch(MyPoint raw){        try{            throw new Exception();        }catch (Exception e){            return raw;        }finally {            raw.x=-2;            raw.y=-2;        }    }}

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