TryCatchFinally.java 的return 问题。

package org.iteye.bbjava.exception;

public class TryCatchFinally {
	public String returnValue;

	public String getStringValue() {

		try {
			if (true)
				return "the value from try ";
			throw new Exception("Exception!");

		} catch (Exception e) {
			return "the value from catch ";
		} finally {
			return "the value from finally ";
		}
	}

	public static void main(String[] args) {
		String str = new TryCatchFinally().getStringValue();
		System.out.println(str);
	}
}

output:

引用

the value from finally

说明最终返回值是从finally块的return语句返回的。其实try块里有返回过值，请看下面：

引用

javac TryCatchFinally.java
javap -v TryCatchFinally

在终端输出：

引用

Compiled from "TryCatchFinally.java"
public class TryCatchFinally extends java.lang.Object
  SourceFile: "TryCatchFinally.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method #11.#27; //  java/lang/Object."<init>":()V
const #2 = String #28; //  the value from try
const #3 = String #29; //  the value from finally
const #4 = class #30; //  java/lang/Exception
const #5 = String #31; //  the value from catch
const #6 = class #32; //  TryCatchFinally
const #7 = Method #6.#27; //  TryCatchFinally."<init>":()V
const #8 = Method #6.#33; //  TryCatchFinally.getStringValue:()Ljava/lang/String;
const #9 = Field #34.#35; //  java/lang/System.out:Ljava/io/PrintStream;
const #10 = Method #36.#37; //  java/io/PrintStream.println:(Ljava/lang/String;)V
const #11 = class #38; //  java/lang/Object
const #12 = Asciz returnValue;
const #13 = Asciz Ljava/lang/String;;
const #14 = Asciz <init>;
const #15 = Asciz ()V;
const #16 = Asciz Code;
const #17 = Asciz LineNumberTable;
const #18 = Asciz getStringValue;
const #19 = Asciz ()Ljava/lang/String;;
const #20 = Asciz StackMapTable;
const #21 = class #30; //  java/lang/Exception
const #22 = class #39; //  java/lang/Throwable
const #23 = Asciz main;
const #24 = Asciz ([Ljava/lang/String;)V;
const #25 = Asciz SourceFile;
const #26 = Asciz TryCatchFinally.java;
const #27 = NameAndType #14:#15;//  "<init>":()V
const #28 = Asciz the value from try ;
const #29 = Asciz the value from finally ;
const #30 = Asciz java/lang/Exception;
const #31 = Asciz the value from catch ;
const #32 = Asciz TryCatchFinally;
const #33 = NameAndType #18:#19;//  getStringValue:()Ljava/lang/String;
const #34 = class #40; //  java/lang/System
const #35 = NameAndType #41:#42;//  out:Ljava/io/PrintStream;
const #36 = class #43; //  java/io/PrintStream
const #37 = NameAndType #44:#45;//  println:(Ljava/lang/String;)V
const #38 = Asciz java/lang/Object;
const #39 = Asciz java/lang/Throwable;
const #40 = Asciz java/lang/System;
const #41 = Asciz out;
const #42 = Asciz Ljava/io/PrintStream;;
const #43 = Asciz java/io/PrintStream;
const #44 = Asciz println;
const #45 = Asciz (Ljava/lang/String;)V;

{
public java.lang.String returnValue;

public TryCatchFinally();
  Code:
   Stack=1, Locals=1, Args_size=1
   0: aload_0
   1: invokespecial #1; //Method java/lang/Object."<init>":()V
   4: return
  LineNumberTable:
   line 1: 0


public java.lang.String getStringValue();
  Code:
   Stack=1, Locals=4, Args_size=1
   0: ldc #2; //String the value from try
   2: astore_1
   3: ldc #3; //String the value from finally
   5: areturn
   6: astore_1
   7: ldc #5; //String the value from catch
   9: astore_2
   10: ldc #3; //String the value from finally
   12: areturn
   13: astore_3
   14: ldc #3; //String the value from finally
   16: areturn
  Exception table:
   from   to  target type
     0     3     6   Class java/lang/Exception

     0     3    13   any
     6    10    13   any
    13    14    13   any
  LineNumberTable:
   line 8: 0
   line 14: 3
   line 11: 6
   line 12: 7
   line 14: 10

  StackMapTable: number_of_entries = 2
   frame_type = 70 /* same_locals_1_stack_item */
     stack = [ class java/lang/Exception ]
   frame_type = 70 /* same_locals_1_stack_item */
     stack = [ class java/lang/Throwable ]


public static void main(java.lang.String[]);
  Code:
   Stack=2, Locals=2, Args_size=1
   0: new #6; //class TryCatchFinally
   3: dup
   4: invokespecial #7; //Method "<init>":()V
   7: invokevirtual #8; //Method getStringValue:()Ljava/lang/String;
   10: astore_1
   11: getstatic #9; //Field java/lang/System.out:Ljava/io/PrintStream;
   14: aload_1
   15: invokevirtual #10; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   18: return
  LineNumberTable:
   line 19: 0
   line 20: 11
   line 21: 18


}

整理中……待续.

附上：try -catch-finally,try-finally 的组合是不一样的，比如：

引用

	static int f() {// 这里报编译错误：must return a resutl of type int!
		int id = 0;
		try {
			return id;
		} catch (Exception e) {

		} finally {

		}
	}

	static int f2() {// 但是这里怎么不报 错误呢？
		int id = 0;
		try {
			return id;
		} finally {
		}
	}

参考资料：1.http://topic.csdn.net/u/20091211/15/a8228a73-f2fe-40c0-92f2-1482224f41dd.html
2.http://download.oracle.com/javase/1.4.2/docs/tooldocs/windows/javap.html