前言
备受争议的Lombok，有的人喜欢它让代码更整洁，有的人不喜欢它，巴拉巴拉一堆原因。在我看来Lombok唯一的缺点可能就是需要安装插件了，但是对于业务开发的项目来说，它的优点远远超过缺点。
@Builder的使用
使用@Builder修饰类
@Data
@Builder
public class UserDO {
private Long id;
private String name;
}
使用建造者模式创建类
@Test
public void test() {
UserDO userDO = UserDO.builder()
.id(1L)
.name("iisheng")
.build();
System.out.println(userDO);
}
编译后源码
执行javac -cp ~/lombok.jar UserDO.java -verbose将.java编译成.class文件。
通过IDE查看该.class源码
public class UserDO {
private Long id;
private String name;
public String toString() {
return "UserDO(id="
+ this.getId() + ", name=" + this.getName() + ")";
}
UserDO(Long var1, String var2) {
this.id = var1;
this.name = var2;
}
public static UserDO.UserDOBuilder builder() {
return new UserDO.UserDOBuilder();
}
private UserDO() {
}
public static class UserDOBuilder {
private Long id;
private String name;
UserDOBuilder() {
}
public UserDO.UserDOBuilder id(Long var1) {
this.id = var1;
return this;
}
public UserDO.UserDOBuilder name(String var1) {
this.name = var1;
return this;
}
public UserDO build() {
return new UserDO(this.id, this.name);
}
}
}
由此，我们可以看出来Builder的实现步骤：
- 在
UserDO中创建静态UserDOBuilder - 编写设置属性方法，返回
UserDOBuilder对象 - 编写
build()方法，返回UserDO对象
是不是很简单？我曾经看过不知道哪个大佬说的一句话，整洁的代码不是说，行数更少，字数更少，而是阅读起来逻辑更清晰。所以，我觉得，哪怕我们不用@Builder，也应该多用这种建造者模式。
建造者模式
UML类图
产品类
public class Product {
private String name;
private Integer val;
Product(String name, Integer val) {
this.name = name;
this.val = val;
}
@Override
public String toString() {
return "Product is " + name + " value is " + val;
}
}
抽象建造者
public abstract class Builder {
protected Integer val;
protected String name;
// 设置产品不同部分，以获得不同的产品
public abstract void setVal(Integer val);
// 设置名字 公用方法
public void setName(String name) {
this.name = name;
}
// 建造产品
public abstract Product buildProduct();
}
具体建造者
public class ConcreteBuilder extends Builder {
@Override
public void setVal(Integer val) {
/**
* 产品类内部的逻辑
* 实际存储的值是 val + 100
*/
this.val = val + 100;
}
@Override
// 组建一个产品
public Product buildProduct() {
// 这块还可以写特殊的校验逻辑
return new Product(name, val);
}
}
导演类
public class Director {
private Builder builder = new ConcreteBuilder();
public Product getAProduct() {
// 设置不同的零件，产生不同的产品
builder.setName("ProductA");
builder.setVal(2);
return builder.buildProduct();
}
}
Product的创建，也依赖于Builder。代码只需要将上面的Product和ConcreteBuilder调整一下即可。
调整后的产品类
public class Product {
private String name;
private Integer val;
Product(Builder builder) {
this.name = builder.name;
this.val = builder.val;
}
@Override
public String toString() {
return "Product is " + name + " value is " + val;
}
}
这代码只是将构造方法改了，使用Builder来创建Product对象。
调整后的具体建造者
public class ConcreteBuilder extends Builder {
@Override
public void setVal(Integer val) {
/**
* 产品类内部的逻辑
* 实际存储的值是 val + 100
*/
this.val = val + 100;
}
@Override
// 组建一个产品
public Product buildProduct() {
// 这块还可以写特殊的校验逻辑
return new Product(this);
}
}
相应的使用带Builder的Product的构造方法。
JDK中的建造者模式
抽象建造者
abstract class AbstractStringBuilder implements Appendable, CharSequence {
/**
* The value is used for character storage.
*/
char[] value;
/**
* The count is the number of characters used.
*/
int count;
public AbstractStringBuilder append(String str) {
if (str == null)
return appendNull();
int len = str.length();
ensureCapacityInternal(count + len);
str.getChars(0, len, value, count);
count += len;
return this;
}
// Documentation in subclasses because of synchro difference
@Override
public AbstractStringBuilder append(CharSequence s) {
if (s == null)
return appendNull();
if (s instanceof String)
return this.append((String)s);
if (s instanceof AbstractStringBuilder)
return this.append((AbstractStringBuilder)s);
return this.append(s, 0, s.length());
}
public AbstractStringBuilder delete(int start, int end) {
if (start < 0)
throw new StringIndexOutOfBoundsException(start);
if (end > count)
end = count;
if (start > end)
throw new StringIndexOutOfBoundsException();
int len = end - start;
if (len > 0) {
System.arraycopy(value, start+len, value, start, count-end);
count -= len;
}
return this;
}
}
具体建造者
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
@Override
public StringBuilder append(String str) {
super.append(str);
return this;
}
@Override
public StringBuilder append(CharSequence s) {
super.append(s);
return this;
}
/**
* @throws StringIndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder delete(int start, int end) {
super.delete(start, end);
return this;
}
}
StringBuilder中的建造者模式比较简单，但是我的确没找到StringBuilder非要用建造者模式的原因，或许就是想让我们写下面这样的代码？
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
sb.append("Love ")
.append("iisheng !")
.insert(0, "I ");
System.out.println(sb);
}
Guava Cache中的建造者模式
如何使用 Guava Cache？
public static void main(String[] args) {
LoadingCache<String, Integer> cache = CacheBuilder.newBuilder()
// 最多存放十个数据
.maximumSize(10)
// 缓存10秒
.expireAfterWrite(10, TimeUnit.SECONDS)
.build(new CacheLoader<String, Integer>() {
// 默认返回-1，也可以是查询操作，如从DB查询
@Override
public Integer load(String key) throws Exception {
return -1;
}
});
// 只查询缓存，没有命中，即返回 null
System.out.println(cache.getIfPresent("key1"));
// put数据，放在缓存中
cache.put("key1", 1);
// 再次查询，已经存在缓存中
System.out.println(cache.getIfPresent("key1"));
//查询缓存，未命中，调用load方法，返回 -1
try {
System.out.println(cache.get("key2"));
} catch (ExecutionException e) {
e.printStackTrace();
}
}
产品接口
@DoNotMock("Use CacheBuilder.newBuilder().build()")
@GwtCompatible
public interface Cache<K, V> {
@Nullable
V getIfPresent(@CompatibleWith("K") Object key);
V get(K key, Callable<? extends V> loader) throws ExecutionException;
void put(K key, V value);
long size();
ConcurrentMap<K, V> asMap();
void cleanUp();
}
另一个产品接口
@GwtCompatible
public interface LoadingCache<K, V> extends Cache<K, V>, Function<K, V> {
V get(K key) throws ExecutionException;
V getUnchecked(K key);
void refresh(K key);
@Deprecated
@Override
V apply(K key);
@Override
ConcurrentMap<K, V> asMap();
}
产品实现类
static class LocalManualCache<K, V> implements Cache<K, V>, Serializable {
final LocalCache<K, V> localCache;
LocalManualCache(CacheBuilder<? super K, ? super V> builder) {
this(new LocalCache<K, V>(builder, null));
}
private LocalManualCache(LocalCache<K, V> localCache) {
this.localCache = localCache;
}
// Cache methods
@Override
public @Nullable V getIfPresent(Object key) {
return localCache.getIfPresent(key);
}
@Override
public V get(K key, final Callable<? extends V> valueLoader) throws ExecutionException {
checkNotNull(valueLoader);
return localCache.get(
key,
new CacheLoader<Object, V>() {
@Override
public V load(Object key) throws Exception {
return valueLoader.call();
}
});
}
@Override
public void put(K key, V value) {
localCache.put(key, value);
}
@Override
public long size() {
return localCache.longSize();
}
@Override
public ConcurrentMap<K, V> asMap() {
return localCache;
}
@Override
public void cleanUp() {
localCache.cleanUp();
}
// Serialization Support
private static final long serialVersionUID = 1;
Object writeReplace() {
return new ManualSerializationProxy<>(localCache);
}
}
另一个产品实现类
static class LocalLoadingCache<K, V> extends LocalManualCache<K, V>
implements LoadingCache<K, V> {
LocalLoadingCache(
CacheBuilder<? super K, ? super V> builder, CacheLoader<? super K, V> loader) {
super(new LocalCache<K, V>(builder, checkNotNull(loader)));
}
// LoadingCache methods
@Override
public V get(K key) throws ExecutionException {
return localCache.getOrLoad(key);
}
@Override
public V getUnchecked(K key) {
try {
return get(key);
} catch (ExecutionException e) {
throw new UncheckedExecutionException(e.getCause());
}
}
@Override
public void refresh(K key) {
localCache.refresh(key);
}
@Override
public final V apply(K key) {
return getUnchecked(key);
}
// Serialization Support
private static final long serialVersionUID = 1;
@Override
Object writeReplace() {
return new LoadingSerializationProxy<>(localCache);
}
}
实际产品实现类LocalCache
上面两个产品类实际上，内部使用的是LocalCache来存储数据。我们再看下LocalCache的实现。
LocalCache继承AbstractCache，我们先看AbstractCache：
@GwtCompatible
public abstract class AbstractCache<K, V> implements Cache<K, V> {
/** Constructor for use by subclasses. */
protected AbstractCache() {}
@Override
public V get(K key, Callable<? extends V> valueLoader) throws ExecutionException {
throw new UnsupportedOperationException();
}
@Override
public void put(K key, V value) {
throw new UnsupportedOperationException();
}
@Override
public void cleanUp() {}
@Override
public long size() {
throw new UnsupportedOperationException();
}
@Override
public ConcurrentMap<K, V> asMap() {
throw new UnsupportedOperationException();
}
}
再来看，LocalCache：
@GwtCompatible(emulated = true)
class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {
/** How long after the last write to an entry the map will retain that entry. */
final long expireAfterWriteNanos;
/** The default cache loader to use on loading operations. */
final @Nullable CacheLoader<? super K, V> defaultLoader;
/**
* Creates a new, empty map with the specified strategy, initial capacity and concurrency level.
*/
LocalCache(
CacheBuilder<? super K, ? super V> builder, @Nullable CacheLoader<? super K, V> loader) {
concurrencyLevel = Math.min(builder.getConcurrencyLevel(), MAX_SEGMENTS);
maxWeight = builder.getMaximumWeight();
weigher = builder.getWeigher();
expireAfterAccessNanos = builder.getExpireAfterAccessNanos();
expireAfterWriteNanos = builder.getExpireAfterWriteNanos();
refreshNanos = builder.getRefreshNanos();
defaultLoader = loader;
int initialCapacity = Math.min(builder.getInitialCapacity(), MAXIMUM_CAPACITY);
if (evictsBySize() && !customWeigher()) {
initialCapacity = (int) Math.min(initialCapacity, maxWeight);
}
// Find the lowest power-of-two segmentCount that exceeds concurrencyLevel, unless
// maximumSize/Weight is specified in which case ensure that each segment gets at least 10
// entries. The special casing for size-based eviction is only necessary because that eviction
// happens per segment instead of globally, so too many segments compared to the maximum size
// will result in random eviction behavior.
int segmentShift = 0;
int segmentCount = 1;
while (segmentCount < concurrencyLevel && (!evictsBySize() || segmentCount * 20 <= maxWeight)) {
++segmentShift;
segmentCount <<= 1;
}
this.segmentShift = 32 - segmentShift;
segmentMask = segmentCount - 1;
this.segments = newSegmentArray(segmentCount);
}
}
建造者
@GwtCompatible(emulated = true)
public final class CacheBuilder<K, V> {
long maximumSize = UNSET_INT;
long expireAfterWriteNanos = UNSET_INT;
Supplier<? extends StatsCounter> statsCounterSupplier = NULL_STATS_COUNTER;
public CacheBuilder<K, V> maximumSize(long maximumSize) {
checkState(
this.maximumSize == UNSET_INT, "maximum size was already set to %s", this.maximumSize);
checkState(
this.maximumWeight == UNSET_INT,
"maximum weight was already set to %s",
this.maximumWeight);
checkState(this.weigher == null, "maximum size can not be combined with weigher");
checkArgument(maximumSize >= 0, "maximum size must not be negative");
this.maximumSize = maximumSize;
return this;
}
public CacheBuilder<K, V> expireAfterWrite(long duration, TimeUnit unit) {
checkState(
expireAfterWriteNanos == UNSET_INT,
"expireAfterWrite was already set to %s ns",
expireAfterWriteNanos);
checkArgument(duration >= 0, "duration cannot be negative: %s %s", duration, unit);
this.expireAfterWriteNanos = unit.toNanos(duration);
return this;
}
public CacheBuilder<K, V> recordStats() {
statsCounterSupplier = CACHE_STATS_COUNTER;
return this;
}
public <K1 extends K, V1 extends V> Cache<K1, V1> build() {
checkWeightWithWeigher();
checkNonLoadingCache();
return new LocalCache.LocalManualCache<>(this);
}
public <K1 extends K, V1 extends V> LoadingCache<K1, V1> build(
CacheLoader<? super K1, V1> loader) {
checkWeightWithWeigher();
return new LocalCache.LocalLoadingCache<>(this, loader);
}
}
LoadingCache接口继承了Cache接口，两个接口都定义了缓存的基本方法CacheLoader是LocalCache的成员变量LocalCache继承AbstractMap，是真正意义上的产品类LocalManualCache是CacheBuilder的build()方法产生的对象的类，LocalManualCache因为有LocalCache作为成员变量，使得它成为了产品类，LocalManualCache实现了Cache接口LocalLoadingCache继承了LocalManualCache，是CacheBuilder的build(CacheLoader<? super K1, V1> loader)方法产生的对象的类，LocalLoadingCache实现了LoadingCache接口
总结
什么时候适合使用建造者模式？
创建一个有很多属性的对象，如果参数在构造方法中写，看起来很乱，一长串不说，还很容易写错。
创建的对象有很多属性是可选择的那种，常见的比如配置类等，不同使用者有不同的配置。
不提供set()方法，使用建造者模式一次性把对象创建完成。
建造者模式和工厂模式的区别是什么？
- 建造者模式，通过设置不同的可选参数，“定制化”的创建不同的对象
- 工厂模式，是直接创建不同但是相关类型的对象（继承同一父类或者接口的一组子类）
最后想说的
由@Builder想到的建造者模式，然后看了StringBuilder以及Guava Cache的源码，其中还是有很多值得我们学习的地方。
建造者模式，可能不同的人有不同的理解，不同的实现有不同的方法，但是我们只有深刻的理解了其中的设计思想，才不至于在项目中生搬硬套，才能灵活运用。