class AbortedConstruction extends Exception {
public AbortedConstruction() {
super( "Construction aborted ");
}
}
class WithCleanup {
private boolean constructed = false;
public WithCleanup(boolean abort)
throws AbortedConstruction {
// Perform construction that might be
// unsucessful (and throw an exception) here.
if(abort) throw new AbortedConstruction();
System.out.println( "After exception ");
constructed = true;
}
public void cleanup() {
System.out.println(
"constructed = " + constructed);
if(constructed == true)
System.out.println( "Cleaning up ");
else
System.out.println(
"Constructor didn 't finish, "+
"not cleaning up ");
}
}
public class E14_CleanupFlag {
public static void main(String args[]) {
WithCleanup wc = null;
try {
wc = new WithCleanup(false);
} catch(AbortedConstruction e) {
System.out.println( "Caught " + e);
} finally {
System.out.println(
"In finally 1, preparing to clean up ");
wc.cleanup();
}
wc = null; // Very important!
try {
try {
wc = new WithCleanup(true);
} catch(AbortedConstruction e) {
System.out.println( "Caught " + e);
} finally {
System.out.println(
"In finally 2, preparing to clean up ");
wc.cleanup();//为什么这里的wc=null,因而不能调用cleanup(),
} //导致抛出java.lang.NullPointerException，