package exceptions;
//: exceptions/InputFile.java
// Paying attention to exceptions in constructors.
import java.io.*;public class InputFile {private BufferedReader in;public InputFile(String fname) throws Exception {try {in = new BufferedReader(new FileReader(fname));// Other code that might throw exceptions} catch(FileNotFoundException e) {System.out.println("Could not open " + fname);// Wasn't open, so don't close it //如果没有打开文件就不需要关闭throw e;} catch(Exception e) {// All other exceptions must close it  如果时其它异常则必须关闭文件try { //in.close()也可能抛出异常,所有要放到try块里面in.close();} catch(IOException e2) {System.out.println("in.close() unsuccessful");}throw e; // Rethrow} finally {  //由于finally总会被执行,如果在这里关闭文件则文件刚打开还没开始使用就关闭了// Don't close it here!!!
    }}public String getLine() {String s;try {s = in.readLine();} catch(IOException e) { //这这异常已被捕获,因此getLine不会抛出任何异常throw new RuntimeException("readLine() failed");//重新抛出新的异常到上层环境,有时会简化编程}return s;}public void dispose() {try {in.close();System.out.println("dispose() successful");} catch(IOException e2) {throw new RuntimeException("in.close() failed");}}
} ///:~

package exceptions;
//: exceptions/Cleanup.java
// Guaranteeing proper cleanup of a resource.public class Cleanup {public static void main(String[] args) {try {InputFile in = new InputFile("Cleanup.java");try {String s;int i = 1;while((s = in.getLine()) != null); // Perform line-by-line processing here...} catch(Exception e) {//这里捕捉的时getLine()方法重新抛出的异常System.out.println("Caught Exception in main");e.printStackTrace(System.out);} finally { //如果构造成功,则一定会执行in.dispose()清理
        in.dispose();}} catch(Exception e) { //InputFile对象在自己的try语句块优先,因此构造失败会进入这里,而不会执行内部的try块的in.colse()System.out.println("InputFile construction failed");}}
} /* Output:
dispose() successful
*///:~

//基本上,你应该仔细考虑所有的可能细节,例如本例的dispose()如果可以抛出异常,那么就需要额外的try语句块package exceptions;
//: exceptions/CleanupIdiom.java
// Each disposable object must be followed by a try-finallyclass NeedsCleanup { // Construction can't failprivate static long counter = 1;private final long id = counter++;public void dispose() {System.out.println("NeedsCleanup " + id + " disposed");}
}class ConstructionException extends Exception {}class NeedsCleanup2 extends NeedsCleanup {// Construction can fail:public NeedsCleanup2() throws ConstructionException {}
}public class CleanupIdiom {public static void main(String[] args) {// Section 1:NeedsCleanup nc1 = new NeedsCleanup();try {// ...} finally {nc1.dispose();}// Section 2:// If construction cannot fail you can group objects:// nc5 constructor 如果对象构造不能失败就不需要任何catch//不能失败的对象构造器对象可以群众在一起NeedsCleanup nc2 = new NeedsCleanup();NeedsCleanup nc3 = new NeedsCleanup();try {// ...} finally {nc3.dispose(); // Reverse order of construction
      nc2.dispose();}// Section 3:// If construction can fail you must guard each one:try {NeedsCleanup2 nc4 = new NeedsCleanup2();try {NeedsCleanup2 nc5 = new NeedsCleanup2();try { //如果nc5对象构造失败则会调用try块清理,否则永不调用// ...} finally {nc5.dispose();}} catch(ConstructionException e) { System.out.println(e);} finally {nc4.dispose();}} catch(ConstructionException e) { // nc4 constructor
      System.out.println(e);}}
} /* Output:
NeedsCleanup 1 disposed
NeedsCleanup 3 disposed
NeedsCleanup 2 disposed
NeedsCleanup 5 disposed
NeedsCleanup 4 disposed
*///:~