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Method chaining is a useful design pattern for building an object and setting its optional fields. A class that supports method chaining provides several setter methods that each return the this reference. However, in a multithreaded environment, a thread may observe shared fields to contain inconsistent values. This noncompliant code example shows the Javabeans JavaBeans pattern which is not safe for multithreaded use.
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final class USCurrency {
// Change requested, denomination (optional fields)
private int quarters = 0;
private int dimes = 0;
private int nickels = 0;
private int pennies = 0;
public USCurrency() {}
// Setter methods
public USCurrency setQuarters(int quantity) {
quarters = quantity;
return this;
}
public USCurrency setDimes(int quantity) {
dimes = quantity;
return this;
}
public USCurrency setNickels(int quantity) {
nickels = quantity;
return this;
}
public USCurrency setPennies(int quantity) {
pennies = quantity;
return this;
}
}
// Client code:
private final USCurrency currency = new USCurrency();
// ...
new Thread(new Runnable() {
@Override public void run() {
currency.setQuarters(1).setDimes(1);
}
}).start();
new Thread(new Runnable() {
@Override public void run() {
currency.setQuarters(2).setDimes(2);
}
}).start();
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The Javabeans JavaBeans pattern uses a no-argument constructor and a series of parallel setter methods to build an object. This pattern is not thread-safe and can lead to inconsistent object state. In this example, a client that constructs a USCurrency object and starts two threads as shown may find the object to contain two quarters and one dime or one quarter and two dimes, contrary to what it expects.
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