tokuhirom's Blog

Java で lazy なアクセサを書きたい

http://projectlombok.org/features/GetterLazy.html

それ lombok でできるよ。

import lombok.Getter;

public class GetterLazyExample {
    @Getter(lazy=true) private final double[] cached = expensive();

    private double[] expensive() {
        double[] result = new double[1000000];
        for (int i = 0; i < result.length; i++) {
            result[i] = Math.asin(i);
        }
        return result;
    }
}

とか書くと

public class GetterLazyExample {
    private final java.util.concurrent.AtomicReference<java.lang.Object> cached = new java.util.concurrent.AtomicReference<java.lang.Object>();
    
    public double[] getCached() {
        java.lang.Object value = this.cached.get();
        if (value == null) {
            synchronized(this.cached) {
                value = this.cached.get();
                if (value == null) {
                    final double[] actualValue = expensive();
                    value = actualValue == null ? this.cached : actualValue;
                    this.cached.set(value);
                }
            }
        }
        return (double[])(value == this.cached ? null : value);
    }
    
    private double[] expensive() {
        double[] result = new double[1000000];
        for (int i = 0; i < result.length; i++) {
            result[i] = Math.asin(i);
        }
        return result;
    }
}

となる、と。。

めっちゃ便利だな!