红豆生南国，春来发几枝。愿君多采撷，此物最相思。
（原创不容易，引用请标记，谢谢）
try catch为一般用于java中的异常处理，
1：try()的用法：
try（）是1.7新增的用法，用来自动关闭括号内能关闭的资源，而不必要手动关闭，其必须实现autocloseable接口，
such as下列代码，MySource是资源类，继承了AutoCloseable接口
public class Myresource implements AutoCloseable{
@Override
public void close() throws Exception {
System.out.println("正在执行关闭方法");
}
public void throwTest(int k) throws Exception{
System.out.println("正在执行方法throwTest");
if(k>5){
throw new Exception();
}
}
}而我测试类这样写时：
public class TryTest {
public static void main(String[] args) {
try{
Myresource myresource = new Myresource();
myresource.throwTest(4);
}catch (Exception e) {
e.printStackTrace();
}
}
}执行结果为：
但是当我测试类这样写时：
public static void main(String[] args) {
try (Myresource myresource = new Myresource()) {
myresource.throwTest(4);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}运行结果为：
将自动关闭资源，测试了下上面资源类继承closeable接口时，采用下面的方法，运行结果也会自动关闭资源，
2：try的返回值问题
当try catch final 中都有return的时候怎么返回呢，写demo
public class TryTest {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 执行");
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 执行");
return string;
}finally {
string = "finally model";
System.out.println("finally 执行");
}
}
}执行结果为：
预料中执行finally已经把值改变为finally 了 但是返回的确实try
javap -verbose一下：
Code:
stack=2, locals=4, args_size=0
0: ldc #34 // String
2: astore_0
3: ldc #36 // String try model
5: astore_0
6: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
9: ldc #38 // String try 执行
11: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
14: aload_0
15: astore_3
16: ldc #40 // String finally model
18: astore_0
19: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
22: ldc #42 // String finally 执行
24: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
27: aload_3
28: areturn
29: astore_1
30: ldc #44 // String catch model
32: astore_0
33: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
36: ldc #46 // String catch 执行
38: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
41: aload_0
42: astore_3
43: ldc #40 // String finally model
45: astore_0
46: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
49: ldc #42 // String finally 执行
51: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
54: aload_3
55: areturn
56: astore_2
57: ldc #40 // String finally model
59: astore_0
60: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
63: ldc #42 // String finally 执行
65: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
68: aload_2
69: athrow第0-2给string0赋值“”,并储存
第3-5 给string0赋值“try model“，并储存
第6-11 调用静态方法print并打印“try 执行”
第14-15 把string0的值放进另一个string3
第16-18 给string0赋值“finally model”
第19-24 打印“String finally 执行”
第27-28 加载string3(当前是以前的string0即“try model”)，然后返回
由此可知，实际返回的是try model，在执行try之前会有一个临时变量String3将string0装进去，然后返回。
当在finally中返回的时候呢?代码如下：
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 执行");
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 执行");
return string;
}finally {
string = "finally model";
System.out.println("finally 执行");
return string;
}
}
}得到的结果为：
同理分析javap 皮一下
Code:
stack=2, locals=2, args_size=0
0: ldc #34 // String
2: astore_0
3: ldc #36 // String try model
5: astore_0
6: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
9: ldc #38 // String try 执行
11: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
14: goto 33
17: astore_1
18: ldc #40 // String catch model
20: astore_0
21: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
24: ldc #42 // String catch 执行
26: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
29: goto 33
32: pop
33: ldc #44 // String finally model
35: astore_0
36: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
39: ldc #46 // String finally 执行
41: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
44: aload_0
45: areturn同上面的分析，不同的是14行的时候就goto到33,33-35直接把“finally model”装进string0
然后打印“finally 执行” 最后aload加载string0 然后返回，即返回的是“finally model”
万一遇见个错误呢怎么返回，代码一下：
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 执行");
int k = 5/0;
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 执行");
return string;
}finally {
string = "finally model";
System.out.println("finally 执行");
}
}
}返回的结果为：
皮一下：
Code:
stack=2, locals=4, args_size=0
0: ldc #34 // String
2: astore_0
3: ldc #36 // String try model
5: astore_0
6: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
9: ldc #38 // String try 执行
11: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
14: iconst_5
15: iconst_0
16: idiv
17: istore_1
18: aload_0
19: astore_3
20: ldc #40 // String finally model
22: astore_0
23: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
26: ldc #42 // String finally 执行
28: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
31: aload_3
32: areturn
33: astore_1
34: ldc #44 // String catch model
36: astore_0
37: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
40: ldc #46 // String catch 执行
42: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
45: aload_0
46: astore_3
47: ldc #40 // String finally model
49: astore_0
50: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
53: ldc #42 // String finally 执行
55: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
58: aload_3
59: areturn
60: astore_2
61: ldc #40 // String finally model
63: astore_0
64: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
67: ldc #42 // String finally 执行
69: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
72: aload_2
73: athrow分析分析，前面讲过的不说，第14-16行除法，存进寄存器1中，遇见异常还是直接跳过第一个return还是咋的
跳过第一个return不说（具体原因我也不清楚），34-46 将string0标记为“catch model”然后打印“catch 执行”然后装进寄存器3中，其他同上面1的里，然后的还是寄存器3里面的值。
在异常的时候 finally加return
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 执行");
int k = 5/0;
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 执行");
return string;
}finally {
string = "finally model";
System.out.println("finally 执行");
return string;
}
}
}这个就不说了 和第二个类似 返回finally model
在catch里面遇见异常呢：
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 执行");
int k = 5/0;
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 执行");
int k = 5/0;
return string;
}finally {
string = "finally model";
System.out.println("finally 执行");
}
}
}直接返回异常
在finally的时候返回呢？
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 执行");
int k = 5/0;
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 执行");
int k = 5/0;
return string;
}finally {
string = "finally model";
System.out.println("finally 执行");
return string;
}
}
}运行结果：
总结总结：在finally中返回，必为返回值finally
如果在catch中抓取的不是那个异常，或者catch中遇见异常，或者finally中遇见异常直接抛出异常
try中异常 返回的是try中的值
so：千万不要在finally中return