Enusre Chained Methods are thread-safe

https://www.securecoding.cert.org/confluence/display/java/VNA04-J.+Ensure+that+calls+to+chained+methods+are+atomic

在上面這篇講Java安全代碼的文章中提到兩段代碼。第一不是線程安全的；第二個是。

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:
class ExampleClientCode {
  private final USCurrency currency = new USCurrency();
  // ...
  public ExampleClientCode() {

  Thread t1 = new Thread(new Runnable() {
        @Override public void run() {
        currency.setQuarters(1).setDimes(1);
        }
  });
  t1.start();

  Thread t2 = new Thread(new Runnable() {
        @Override public void run() {
          currency.setQuarters(2).setDimes(2);
        }
    });
    t2.start();
  }
}



final class USCurrency {
  private final int quarters;
  private final int dimes;
  private final int nickels;
  private final int pennies;

  public USCurrency(Builder builder) {
    this.quarters = builder.quarters;
    this.dimes = builder.dimes;
    this.nickels = builder.nickels;
    this.pennies = builder.pennies;
  }

  // Static class member
  public static class Builder {
    private int quarters = 0;
    private int dimes = 0;
    private int nickels = 0;
    private int pennies = 0;
    public static Builder newInstance() {
      return new Builder();
    }

    private Builder() {}

    // Setter methods
    public Builder setQuarters(int quantity) {
      this.quarters = quantity;
      return this;
    }
    public Builder setDimes(int quantity) {
      this.dimes = quantity;
      return this;
    }
    public Builder setNickels(int quantity) {
      this.nickels = quantity;
      return this;
    }
    public Builder setPennies(int quantity) {
      this.pennies = quantity;
    return this;
    }

    public USCurrency build() {
      return new USCurrency(this);
    }
  }
}

// Client code:
class ExampleClientCode? {

  private volatile USCurrency currency;
  // ...

  public ExampleClientCode() {

    Thread t1 = new Thread(new Runnable() {
      @Override public void run() {
        currency = USCurrency.Builder.newInstance().
                    setQuarters(1).setDimes(1).build();
        }
    });
    t1.start();

    Thread t2 = new Thread(new Runnable() {
        @Override public void run() {
           currency = USCurrency.Builder.newInstance().
                   setQuarters(2).setDimes(2).build();
    }
    });
    t2.start();

//...
}
}

第二個在<<Effective Java 2nd>>中提到，是Item2。我理解第二個所以是線程安全是因爲兩個線程構建了兩個不同的對象，也就是兩個線程沒有訪問同一個對象。對第一個程序做類似的處理也能達到線程安全的目的。

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:
class ExampleClientCode {
  private final USCurrency currency;
  // ...
  public ExampleClientCode() {

  Thread t1 = new Thread(new Runnable() {
        @Override public void run() {
          USCurrency temp = new USCurrency();
          temp.setQuarters(1).setDimes(1);
          currency = temp;
        }
  });
  t1.start();

  Thread t2 = new Thread(new Runnable() {
        @Override public void run() {
          USCurrency temp = new USCurrency();
          temp.setQuarters(2).setDimes(2);
          currency = temp;
        }
    });
    t2.start();
  }
}