java基础-finally关键字

程序不被中断(JVM进程直接死掉)的情况，总是执行的

没有返回值的情况

/**
	 * 	没有返回值的情况下
	 */
	public static void method1() {
		try {
			int i = 1 / 0;
		} catch (Exception e) {
			e.printStackTrace();
			System.out.println("catch");
		}finally {
			System.out.println("finally");
		}
	}

	//输出结果
	java.lang.ArithmeticException: / by zero
	at com.dyy.FinallyDemo.method1(FinallyDemo.java:13)
	at com.dyy.FinallyDemo.main(FinallyDemo.java:6)
catch
finally

           

有返回值的情况下

return在外层

/**
	 * 	有返回值的情况下 return在外层
	 */
	public static int method2() {
		int k = 0; 
		try {
			int i = 1 / 0;
		} catch (Exception e) {
			e.printStackTrace();
			System.out.println("catch");
		}finally {
			System.out.println("finally");
			k = 1 ;
		}
		return k;
	}
	
	//输出
	java.lang.ArithmeticException: / by zero
	at com.dyy.FinallyDemo.method2(FinallyDemo.java:28)
	at com.dyy.FinallyDemo.main(FinallyDemo.java:6)
catch
finally
1
           

return在内层

/**
	 * 	没有返回值的情况下 return在内层
	 */
	public static int method3() {
		int k = 0; 
		try {
			++k;
			int i = 1 / 0;
			++k;
			return k;
		} catch (Exception e) {
			e.printStackTrace();
			++k;
			System.out.println("catch");
		}finally {
			++k;
			System.out.println("finally");
		}
		return k;
	}

 //输出结果
 java.lang.ArithmeticException: / by zero
	at com.dyy.FinallyDemo.method3(FinallyDemo.java:46)
	at com.dyy.FinallyDemo.main(FinallyDemo.java:6)
catch
finally
3
           

说明finally总是会执行的

使用场景

锁资源释放

public void put(Object object) throws InterruptedException {
        final ReentrantLock lock = this.lock;
        //获取独占锁 可中断的
        lock.lockInterruptibly();
        try {
        	//具体对共享资源的操作
        } finally {
        	//释放独占锁 
            lock.unlock();
        }
    }
           

流资源的处理

public static void method4(){
		File file = new File("/xx");
		FileInputStream fileInputStream = null;
		try {
			 fileInputStream = new FileInputStream(file);
			 byte[] bytes = new byte[1024];
			 StringBuilder sb = new StringBuilder();
			 while (fileInputStream.read(bytes) != -1) {
				 sb.append(new String(bytes));
			}
			System.out.println("result=" + sb.toString());
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}finally{
			if(fileInputStream != null){
				try {
					fileInputStream.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			
		}
	}
           

连接的释放

public static void metohd5(){
		Connection connection = null;
		try {
			Class.forName("com.mysql.jdbc.Driver");
			connection = DriverManager.getConnection("xxx:3306","username","password");
			//具体操作
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}finally {
			if(connection != null){
				try {
					connection.close();
				} catch (SQLException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
		}	
           

总结

对一些有限的资源，我们在使用完毕之后一定要记得关闭掉，避免都资源造成浪费，对于一些经常会使用到的资源，我们总是需要打开使用然后关闭，也就有了池的概念，把一些资源放入池中，不关闭掉，使用的时候可以直接获取，使用完之后放回池中就可以了。