Java异常处理（二）

Java异常处理（二）

Java基本的异常处理机制已经在上一篇的文章中写出，这篇文章主要分析在try{ ... }catch{ ... }代码块中加return语句

后，对finally代码块的影响及对其调用者的影响。

下面看这段代码：

public class TestException {
public TestException() {
}

boolean testEx() throws Exception {
boolean ret = true;
try {
ret = testEx1();
} catch (Exception e) {
System.out.println("testEx, catch exception");
ret = false;
throw e;
} finally {
System.out.println("testEx, finally; return value=" + ret);
return ret;
}
}

boolean testEx1() throws Exception {
boolean ret = true;
try {
ret = testEx2();
if (!ret) {
return false;
}
System.out.println("testEx1, at the end of try");
return ret;
} catch (Exception e) {
System.out.println("testEx1, catch exception");
ret = false;
throw e;
} finally {
System.out.println("testEx1, finally; return value=" + ret);
return ret;
}
}

boolean testEx2() throws Exception {
boolean ret = true;
try {
int b = 12;
int c;
for (int i = 2; i >= -2; i--) {
c = b / i;
System.out.println("i=" + i);
}
return true;
} catch (Exception e) {
System.out.println("testEx2, catch exception");
ret = false;
throw e;
} finally {
System.out.println("testEx2, finally; return value=" + ret);
return ret;  //会屏蔽Exception
}
}

public static void main(String[] args) {
TestException testException1 = new TestException();
try {
testException1.testEx();
} catch (Exception e) {
e.printStackTrace();
}
}
}

其运行结果如下：（和你想象的是否一致呢？）

i=2
i=1
testEx2, catch exception
testEx2, finally; return value=false
testEx1, finally; return value=false
testEx, finally; return value=false

可以用Eclipse进行Step Into进行逐步调试，会有意想不到的效果。

在看看下面这段代码：

public class TestException {
public TestException() {
}

boolean testEx() throws Exception {
boolean ret = true;
try {
ret = testEx1();
} catch (Exception e) {
System.out.println("testEx, catch exception");
ret = false;
throw e;
} finally {
System.out.println("testEx, finally; return value=" + ret);
return ret;
}
}

boolean testEx1() throws Exception {
boolean ret = true;
try {
ret = testEx2();
if (!ret) {
return false;
}
System.out.println("testEx1, at the end of try");
return ret;
} catch (Exception e) {
System.out.println("testEx1, catch exception");
ret = false;
throw e;
} finally {
System.out.println("testEx1, finally; return value=" + ret);
return ret;
}
}

boolean testEx2() throws Exception {
boolean ret = true;
try {
int b = 12;
int c;
for (int i = 2; i >= -2; i--) {
c = b / i;
System.out.println("i=" + i);
}
return true;
} catch (Exception e) {
System.out.println("testEx2, catch exception");
ret = false;
throw e;
} finally {
System.out.println("testEx2, finally; return value=" + ret);
// return ret;  //会屏蔽Exception
}
}

public static void main(String[] args) {
TestException testException1 = new TestException();
try {
testException1.testEx();
} catch (Exception e) {
e.printStackTrace();
}
}
}

得到的运行结果如下：

i=2
i=1
testEx2, catch exception
testEx2, finally; return value=false
testEx1, catch exception
testEx1, finally; return value=false
testEx, finally; return value=false

对比以上两段代码，不难发现，两个的主要区别只是一行语句的差别在于testEx2方法下最后一句：return ret;

通过调试不难得出这样的结论：即使在方法内捕捉到异常，如果在finally代码段中加入了返回值，那么异常将会被返回值屏蔽。