At
http://code.google.com/docreader/#p=google-guice&s=google-guice&t=InjectingProviders
they say:
"With normal dependency injection, each type gets exactly *one
instance* of each of its dependent types. The RealBillingService gets
one CreditCardProcessor and one TransactionLog. When this flexibility
is necessary, Guice binds a provider."
But CreditCardProcessor and TransactionLog *are* two different types
(and therefore only need one instance each). Why is this "flexibility"
not "normal"?
Furthermore, down the page under "Providers for Multiple Instances",
they give this code:
public class LogFileTransactionLog implements TransactionLog {
private final Provider<LogFileEntry> logFileProvider;
@Inject
public LogFileTransactionLog(Provider<LogFileEntry> logFileProvider)
{
this.logFileProvider = logFileProvider;
}
public void logChargeResult(ChargeResult result) {
LogFileEntry summaryEntry = logFileProvider.get();
summaryEntry.setText("Charge " + (result.wasSuccessful() ?
"success" : "failure"));
summaryEntry.save();
if (!result.wasSuccessful()) {
LogFileEntry detailEntry = logFileProvider.get();
detailEntry.setText("Failure result: " + result);
detailEntry.save();
}
}
Why does LogFileEntry need a provider? Wouldn't this work just fine
without one:
public class LogFileTransactionLog implements TransactionLog {
private final LogFileEntry logFileEntry;
@Inject
public LogFileTransactionLog(LogFileEntry logFileEntry) {
this.logFileEntry = logFileEntry;
}
public void logChargeResult(ChargeResult result) {
logFileEntry.setText("Charge " + (result.wasSuccessful() ?
"success" : "failure"));
logFileEntry.save();
if (!result.wasSuccessful()) {
logFileEntry.setText("Failure result: " + result);
logFileEntry.save();
}
}
What am I missing? Does each call to Provider.get() create a new
instance of LogFileEntry? If so, how would you gather them up later?
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