// Double-check idiom for lazy initialization of instance fields
private volatile FieldType field;
private FieldType getField() {
FieldType result = field;
if (result == null) { // First check (no locking)
synchronized (this) {
if (field == null) // Second check (with locking)
field = result = computeFieldType();
}
}
return result;
}
这里是原书第三版的第 83 条：
这里面的局部变量 result 不是很好理解，而且我在 JDK 21 下跑出来是可能取到 null 的，求大佬解释一下~
破案了，就是错的，看这里https://github.com/jbloch/effective-java-3e-source-code/issues/8，issues 里面给出了解决方法：
private FieldType getField() {
FieldType result = field;
if (result == null) { // First check (no locking)
synchronized (this) {
if (field == null) // Second check (with locking)
field = result = computeFieldValue();
else
result = field;
}
}
return result;
}
result, Field, null, fieldtype
