我创建这个类是为了不可变并且具有流畅的 API：
public final class Message {
public final String email;
public final String escalationEmail;
public final String assignee;
public final String conversationId;
public final String subject;
public final String userId;
public Message(String email, String escalationEmail, String assignee, String conversationId, String subject, String userId) {
this.email = email;
this.escalationEmail = escalationEmail;
this.assignee = assignee;
this.conversationId = conversationId;
this.subject = subject;
this.userId = userId;
}
public Message() {
email = "";
escalationEmail = "";
assignee = "";
conversationId = "";
subject = "";
userId = "";
}
public Message email(String e) { return new Message(e, escalationEmail, assignee, conversationId, subject, userId); }
public Message escalationEmail(String e) { return new Message(email, e, assignee, conversationId, subject, userId); }
public Message assignee(String a) { return new Message(email, escalationEmail, a, conversationId, subject, userId); }
public Message conversationId(String c) { return new Message(email, escalationEmail, assignee, c, subject, userId); }
public Message subject(String s) { return new Message(email, escalationEmail, assignee, conversationId, s, userId); }
public Message userId(String u) { return new Message(email, escalationEmail, assignee, conversationId, subject, u); }
}
我的问题是，当像这样创建新对象时，优化器是否能够避免大量对象创建：
Message m = new Message()
.email("f[email protected] /cdn-cgi/l/email-protection")
.assignee("[email protected] /cdn-cgi/l/email-protection")
.subject("subj");
制作一个单独的可变构建器对象有什么好处吗？
更新2：阅读 apangin 的答案后，我的基准测试无效。我将其保留在这里，以供参考如何不进行基准测试:)
Update:我冒昧地用这段代码自己测量了这一点：
public final class Message {
public final String email;
public final String escalationEmail;
public final String assignee;
public final String conversationId;
public final String subject;
public final String userId;
public static final class MessageBuilder {
private String email;
private String escalationEmail;
private String assignee;
private String conversationId;
private String subject;
private String userId;
MessageBuilder email(String e) { email = e; return this; }
MessageBuilder escalationEmail(String e) { escalationEmail = e; return this; }
MessageBuilder assignee(String e) { assignee = e; return this; }
MessageBuilder conversationId(String e) { conversationId = e; return this; }
MessageBuilder subject(String e) { subject = e; return this; }
MessageBuilder userId(String e) { userId = e; return this; }
public Message create() {
return new Message(email, escalationEmail, assignee, conversationId, subject, userId);
}
}
public static MessageBuilder createNew() {
return new MessageBuilder();
}
public Message(String email, String escalationEmail, String assignee, String conversationId, String subject, String userId) {
this.email = email;
this.escalationEmail = escalationEmail;
this.assignee = assignee;
this.conversationId = conversationId;
this.subject = subject;
this.userId = userId;
}
public Message() {
email = "";
escalationEmail = "";
assignee = "";
conversationId = "";
subject = "";
userId = "";
}
public Message email(String e) { return new Message(e, escalationEmail, assignee, conversationId, subject, userId); }
public Message escalationEmail(String e) { return new Message(email, e, assignee, conversationId, subject, userId); }
public Message assignee(String a) { return new Message(email, escalationEmail, a, conversationId, subject, userId); }
public Message conversationId(String c) { return new Message(email, escalationEmail, assignee, c, subject, userId); }
public Message subject(String s) { return new Message(email, escalationEmail, assignee, conversationId, s, userId); }
public Message userId(String u) { return new Message(email, escalationEmail, assignee, conversationId, subject, u); }
static String getString() {
return new String("hello");
// return "hello";
}
public static void main(String[] args) {
int n = 1000000000;
long before1 = System.nanoTime();
for (int i = 0; i < n; ++i) {
Message m = new Message()
.email(getString())
.assignee(getString())
.conversationId(getString())
.escalationEmail(getString())
.subject(getString())
.userId(getString());
}
long after1 = System.nanoTime();
long before2 = System.nanoTime();
for (int i = 0; i < n; ++i) {
Message m = Message.createNew()
.email(getString())
.assignee(getString())
.conversationId(getString())
.escalationEmail(getString())
.subject(getString())
.userId(getString())
.create();
}
long after2 = System.nanoTime();
System.out.println("no builder : " + (after1 - before1)/1000000000.0);
System.out.println("with builder: " + (after2 - before2)/1000000000.0);
}
}
我发现如果字符串参数不是新对象，但都是相同的（请参阅 getString 中的注释代码），差异是显着的（构建器更快）
在我想象的更现实的场景中，当所有字符串都是新对象时，差异可以忽略不计，并且 JVM 启动会导致第一个字符串稍微慢一点（我尝试了两种方法）。
使用“新字符串”，代码总共也慢了很多倍（我不得不减少n），也许表明正在对“新消息”进行一些优化，但对“新字符串”没有进行优化。
