[英]Inheritance and Try-With-Resources
假設有兩個類實現了AutoCloseable
接口,如下所示:
public class Closing1 implements AutoCloseable {
private boolean closed;
@Override
public void close() throws Exception {
if (closed) {
throw new Exception("Closed Already");
}
this.closed = true;
System.out.println("Closing1 closed");
}
public boolean isClosed() {
return closed;
}
}
和
public class Closing2 implements AutoCloseable {
private Closing1 cl1;
public Closing2(Closing1 cl1) {
this.cl1 = cl1;
}
@Override
public void close() throws Exception {
if(!cl1.isClosed()) {
throw new Exception("Closing1 not closed");
}
System.out.println("Closing2 closed");
}
}
我發現嘗試資源的所有變化都會導致異常! 我在這里缺少什么,還是只是TWR的設計方式?
try(Closing1 c1 = new Closing1();Closing2 c2 = new Closing2(c1)){
System.out.println("Done");
} //Exception while auto closing C2
要么
try(Closing1 c1 = new Closing1();Closing2 c2 = new Closing2(c1)){
System.out.println("Done");
c1.close();
} // exception while auto closing c1
Try-with-resources將按照聲明的相反順序關閉資源。 這意味着將首先調用c2.close()
,這將在編碼后引發異常。
首先從try-with-resources開始, https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html
如第一個示例所示:
try (BufferedReader br = new BufferedReader(new FileReader(path))) {
return br.readLine();
}
人們不一定會命名鏈中的所有內容。
除非您明確需要c1(關閉以外),否則在現實生活中,您的代碼片段看起來像
try(Closing2 c2 = new Closing2(new Closing1())){
System.out.println("Done");
}
並且您肯定不會在try塊中調用c1.close()
,因為根本沒有c1。
記住這一點,因為所包含的c1沒有關閉而從c2引發異常是完全錯誤的,實際上c2擁有Closing1對象並應在其上調用close()
:
class Close1 implements AutoCloseable {
@Override
public void close() throws Exception {
System.out.println("Closing c1");
}
}
class Close2 implements AutoCloseable {
Close1 c1;
Close2(Close1 c1) {
this.c1=c1;
}
@Override
public void close() throws Exception {
System.out.print("Closing c1 from c2: ");
c1.close();
System.out.println("Closing c2");
}
}
void test() {
System.out.println("Before try block");
try(Close2 c2=new Close2(new Close1())) {
System.out.println("In try block");
}
catch(Exception ex) {
System.out.println("Exception: "+ex);
}
finally {
System.out.println("In finally block");
}
System.out.println("After try block");
}
但是,如果有人給c1命名,它將被關閉兩次,這就是冪等性出現在圖片中的位置,正如某人已經建議的那樣:
System.out.println("Before try block");
try(Close1 c1 = new Close1(); Close2 c2 = new Close2(c1)){
System.out.println("In try block");
}
catch(Exception ex){
System.out.println("Exception: "+ex);
}
finally{
System.out.println("In finally block");
}
System.out.println("After try block");
正如前面提到的BufferedReader
,這是它的close()
方法:
public void close() throws IOException {
synchronized (lock) {
if (in == null)
return;
try {
in.close();
} finally {
in = null;
cb = null;
}
}
}
如果它具有in
,它將被關閉並為空(在finally塊中,因此即使發生異常也會發生),並且所有這些都在線程安全的塊中。 ( cb
只是一個字符數組,它也會被歸零,從而稍微簡化了垃圾收集器的壽命)。 由於將finally塊中的所有內容都設為空,因此對該同一個方法的任何額外調用都不會做任何事情(除了暫時同步鎖之外)。
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