## Chapter 12. Serialization（序列化）
### Item 88: Write readObject methods defensively（防御性地編寫 readObject 方法）
Item 50 contains an immutable date-range class with mutable private Date fields. The class goes to great lengths to preserve its invariants and immutability by defensively copying Date objects in its constructor and accessors. Here is the class:
[Item-50](/Chapter-8/Chapter-8-Item-50-Make-defensive-copies-when-needed.md) 包含一個具有可變私有 Date 字段的不可變日期范圍類。該類通過在構造函數和訪問器中防御性地復制 Date 對象，不遺余力地保持其不變性和不可變性。它是這樣的：
```
// Immutable class that uses defensive copying
public final class Period {
private final Date start;
private final Date end;
/**
* @param start the beginning of the period
* @param end the end of the period; must not precede start
* @throws IllegalArgumentException if start is after end
* @throws NullPointerException if start or end is null
*/
public Period(Date start, Date end) {
this.start = new Date(start.getTime());
this.end = new Date(end.getTime());
if (this.start.compareTo(this.end) > 0)
throw new IllegalArgumentException(start + " after " + end);
}
public Date start () { return new Date(start.getTime()); }
public Date end () { return new Date(end.getTime()); }
public String toString() { return start + " - " + end; }
... // Remainder omitted
}
```
Suppose you decide that you want this class to be serializable. Because the physical representation of a Period object exactly mirrors its logical data content, it is not unreasonable to use the default serialized form (Item 87). Therefore, it might seem that all you have to do to make the class serializable is to add the words implements Serializable to the class declaration. If you did so, however, the class would no longer guarantee its critical invariants.
假設你決定讓這個類可序列化。由于 Period 對象的物理表示精確地反映了它的邏輯數據內容，所以使用默認的序列化形式是合理的（[Item-87](/Chapter-12/Chapter-12-Item-87-Consider-using-a-custom-serialized-form.md)）。因此，要使類可序列化，似乎只需將實現 Serializable 接口。但是，如果這樣做，該類將不再保證它的臨界不變量。
The problem is that the readObject method is effectively another public constructor, and it demands all of the same care as any other constructor. Just as a constructor must check its arguments for validity (Item 49) and make defensive copies of parameters where appropriate (Item 50), so must a readObject method. If a readObject method fails to do either of these things, it is a relatively simple matter for an attacker to violate the class’s invariants.
問題是 readObject 方法實際上是另一個公共構造函數，它與任何其他構造函數有相同的注意事項。如，構造函數必須檢查其參數的有效性（[Item-49](/Chapter-8/Chapter-8-Item-49-Check-parameters-for-validity.md)）并在適當的地方制作防御性副本（[Item-50](/Chapter-8/Chapter-8-Item-50-Make-defensive-copies-when-needed.md)）一樣，readObject 方法也必須這樣做。如果 readObject 方法沒有做到這兩件事中的任何一件，那么攻擊者就很容易違反類的不變性。
Loosely speaking, readObject is a constructor that takes a byte stream as its sole parameter. In normal use, the byte stream is generated by serializing a normally constructed instance. The problem arises when readObject is presented with a byte stream that is artificially constructed to generate an object that violates the invariants of its class. Such a byte stream can be used to create an impossible object, which could not have been created using a normal constructor.
不嚴格地說，readObject 是一個構造函數，它唯一的參數是字節流。在正常使用中，字節流是通過序列化一個正常構造的實例生成的。當 readObject 呈現一個字節流時，問題就出現了，這個字節流是人為構造的，用來生成一個違反類不變性的對象。這樣的字節流可用于創建一個不可思議的對象，而該對象不能使用普通構造函數創建。
Assume that we simply added implements Serializable to the class declaration for Period. This ugly program would then generate a Period instance whose end precedes its start. The casts on byte values whose highorder bit is set is a consequence of Java’s lack of byte literals combined with the unfortunate decision to make the byte type signed:
假設我們只是簡單地讓 Period 實現 Serializable 接口。然后，這個有問題的程序將生成一個 Period 實例，其結束比起始時間還要早。對其高位位設置的字節值進行強制轉換，這是由于 Java 缺少字節字面值，再加上讓字節類型簽名的錯誤決定導致的：
```
public class BogusPeriod {
// Byte stream couldn't have come from a real Period instance!
private static final byte[] serializedForm = {
(byte)0xac, (byte)0xed, 0x00, 0x05, 0x73, 0x72, 0x00, 0x06,
0x50, 0x65, 0x72, 0x69, 0x6f, 0x64, 0x40, 0x7e, (byte)0xf8,
0x2b, 0x4f, 0x46, (byte)0xc0, (byte)0xf4, 0x02, 0x00, 0x02,
0x4c, 0x00, 0x03, 0x65, 0x6e, 0x64, 0x74, 0x00, 0x10, 0x4c,
0x6a, 0x61, 0x76, 0x61, 0x2f, 0x75, 0x74, 0x69, 0x6c, 0x2f,
0x44, 0x61, 0x74, 0x65, 0x3b, 0x4c, 0x00, 0x05, 0x73, 0x74,
0x61, 0x72, 0x74, 0x71, 0x00, 0x7e, 0x00, 0x01, 0x78, 0x70,
0x73, 0x72, 0x00, 0x0e, 0x6a, 0x61, 0x76, 0x61, 0x2e, 0x75,
0x74, 0x69, 0x6c, 0x2e, 0x44, 0x61, 0x74, 0x65, 0x68, 0x6a,
(byte)0x81, 0x01, 0x4b, 0x59, 0x74, 0x19, 0x03, 0x00, 0x00,
0x78, 0x70, 0x77, 0x08, 0x00, 0x00, 0x00, 0x66, (byte)0xdf,
0x6e, 0x1e, 0x00, 0x78, 0x73, 0x71, 0x00, 0x7e, 0x00, 0x03,
0x77, 0x08, 0x00, 0x00, 0x00, (byte)0xd5, 0x17, 0x69, 0x22,
0x00, 0x78
};
public static void main(String[] args) {
Period p = (Period) deserialize(serializedForm);
System.out.println(p);
}
// Returns the object with the specified serialized form
static Object deserialize(byte[] sf) {
try {
return new ObjectInputStream(new ByteArrayInputStream(sf)).readObject();
} catch (IOException | ClassNotFoundException e) {
throw new IllegalArgumentException(e);
}
}
}
```
The byte array literal used to initialize serializedForm was generated by serializing a normal Period instance and hand-editing the resulting byte stream. The details of the stream are unimportant to the example, but if you’re curious, the serialization byte-stream format is described in the Java Object Serialization Specification [Serialization, 6]. If you run this program, it prints Fri Jan 01 12:00:00 PST 1999 - Sun Jan 01 12:00:00 PST 1984. Simply declaring Period serializable enabled us to create an object that violates its class invariants.
用于初始化 serializedForm 的字節數組文本是通過序列化一個普通 Period 實例并手工編輯得到的字節流生成的。流的細節對示例并不重要，但是如果你感興趣，可以在《JavaTM Object Serialization Specification》[serialization, 6]中查到序列化字節流的格式描述。如果你運行這個程序，它將打印 `Fri Jan 01 12:00:00 PST 1999 - Sun Jan 01 12:00:00 PST 1984`。只需聲明 Period 可序列化，就可以創建一個違反其類不變性的對象。
To fix this problem, provide a readObject method for Period that calls defaultReadObject and then checks the validity of the deserialized object. If the validity check fails, the readObject method throws InvalidObjectException, preventing the deserialization from completing:
要解決此問題，請為 Period 提供一個 readObject 方法，該方法調用 defaultReadObject，然后檢查反序列化對象的有效性。如果有效性檢查失敗，readObject 方法拋出 InvalidObjectException，阻止反序列化完成：
```
// readObject method with validity checking - insufficient!
private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
s.defaultReadObject();
// Check that our invariants are satisfied
if (start.compareTo(end) > 0)
throw new InvalidObjectException(start +" after "+ end);
}
```
While this prevents an attacker from creating an invalid Period instance, there is a more subtle problem still lurking. It is possible to create a mutable Period instance by fabricating a byte stream that begins with a valid Period instance and then appends extra references to the private Date fields internal to the Period instance. The attacker reads the Period instance from the ObjectInputStream and then reads the “rogue object references” that were appended to the stream. These references give the attacker access to the objects referenced by the private Date fields within the Period object. By mutating these Date instances, the attacker can mutate the Period instance. The following class demonstrates this attack:
雖然這可以防止攻擊者創建無效的 Period 實例，但還有一個更微妙的問題仍然潛伏著。可以通過字節流來創建一個可變的 Period 實例，該字節流以一個有效的 Period 實例開始，然后向 Period 實例內部的私有日期字段追加額外的引用。攻擊者從 ObjectInputStream 中讀取 Period 實例，然后讀取附加到流中的「惡意對象引用」。這些引用使攻擊者能夠訪問 Period 對象中的私有日期字段引用的對象。通過修改這些日期實例，攻擊者可以修改 Period 實例。下面的類演示了這種攻擊：
```
public class MutablePeriod {
// A period instance
public final Period period;
// period's start field, to which we shouldn't have access
public final Date start;
// period's end field, to which we shouldn't have access
public final Date end;
public MutablePeriod() {
try {
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(bos);
// Serialize a valid Period instance
out.writeObject(new Period(new Date(), new Date()));
/*
* Append rogue "previous object refs" for internal
* Date fields in Period. For details, see "Java
* Object Serialization Specification," Section 6.4.
*/
byte[] ref = { 0x71, 0, 0x7e, 0, 5 }; // Ref #5
bos.write(ref); // The start field
ref[4] = 4; // Ref # 4
bos.write(ref); // The end field
// Deserialize Period and "stolen" Date references
ObjectInputStream in = new ObjectInputStream(new ByteArrayInputStream(bos.toByteArray()));
period = (Period) in.readObject();
start = (Date) in.readObject();
end = (Date) in.readObject();
} catch (IOException | ClassNotFoundException e) {
throw new AssertionError(e);
}
}
}
```
To see the attack in action, run the following program:
要查看攻擊的實際效果，請運行以下程序：
```
public static void main(String[] args) {
MutablePeriod mp = new MutablePeriod();
Period p = mp.period;
Date pEnd = mp.end;
// Let's turn back the clock
pEnd.setYear(78);
System.out.println(p);
// Bring back the 60s!
pEnd.setYear(69);
System.out.println(p);
}
```
In my locale, running this program produces the following output:
在我的語言環境中，運行這個程序會產生以下輸出：
```
Wed Nov 22 00:21:29 PST 2017 - Wed Nov 22 00:21:29 PST 1978
Wed Nov 22 00:21:29 PST 2017 - Sat Nov 22 00:21:29 PST 1969
```
While the Period instance is created with its invariants intact, it is possible to modify its internal components at will. Once in possession of a mutable Period instance, an attacker might cause great harm by passing the instance to a class that depends on Period’s immutability for its security. This is not so farfetched: there are classes that depend on String’s immutability for their security.
雖然創建 Period 實例時保留了它的不變性，但是可以隨意修改它的內部組件。一旦擁有一個可變的 Period 實例，攻擊者可能會將實例傳遞給一個依賴于 Period 的不變性來保證其安全性的類，從而造成極大的危害。這并不是牽強附會的：有些類依賴于 String 的不變性來保證其安全。
The source of the problem is that Period’s readObject method is not doing enough defensive copying. **When an object is deserialized, it is critical to defensively copy any field containing an object reference that a client must not possess.** Therefore, every serializable immutable class containing private mutable components must defensively copy these components in its readObject method. The following readObject method suffices to ensure Period’s invariants and to maintain its immutability:
問題的根源在于 Period 的 readObject 方法沒有進行足夠的防御性復制。**當對象被反序列化時，對任何客戶端不能擁有的對象引用的字段進行防御性地復制至關重要。** 因此，對于每個可序列化的不可變類，如果它包含了私有的可變組件，那么在它的 readObjec 方法中，必須要對這些組件進行防御性地復制。下面的 readObject 方法足以保證周期的不變性，并保持其不變性：
```
// readObject method with defensive copying and validity checking
private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
s.defaultReadObject();
// Defensively copy our mutable components
start = new Date(start.getTime());
end = new Date(end.getTime());
// Check that our invariants are satisfied
if (start.compareTo(end) > 0)
throw new InvalidObjectException(start +" after "+ end);
}
```
Note that the defensive copy is performed prior to the validity check and that we did not use Date’s clone method to perform the defensive copy. Both of these details are required to protect Period against attack (Item 50). Note also that defensive copying is not possible for final fields. To use the readObject method, we must make the start and end fields nonfinal. This is unfortunate, but it is the lesser of two evils. With the new readObject method in place and the final modifier removed from the start and end fields, the MutablePeriod class is rendered ineffective. The above attack program now generates this output:
注意，防御副本是在有效性檢查之前執行的，我們沒有使用 Date 的 clone 方法來執行防御副本。這兩個細節對于保護 Period 免受攻擊是必要的(第50項)。還要注意，防御性復制不可能用于 final 字段。要使用 readObject 方法，必須使 start 和 end 字段非 final。這是不幸的，但卻是權衡利弊后的方案。使用新的 readObject 方法，并從 start 和 end 字段中刪除 final 修飾符，MutablePeriod 類將無效。上面的攻擊程序現在生成這個輸出：
```
Wed Nov 22 00:23:41 PST 2017 - Wed Nov 22 00:23:41 PST 2017
Wed Nov 22 00:23:41 PST 2017 - Wed Nov 22 00:23:41 PST 2017
```
Here is a simple litmus test for deciding whether the default readObject method is acceptable for a class: would you feel comfortable adding a public constructor that took as parameters the values for each nontransient field in the object and stored the values in the fields with no validation whatsoever? If not, you must provide a readObject method, and it must perform all the validity checking and defensive copying that would be required of a constructor. Alternatively, you can use the serialization proxy pattern (Item 90). This pattern is highly recommended because it takes much of the effort out of safe deserialization.
下面是一個簡單的測試，用于判斷默認 readObject 方法是否可用于類：你是否愿意添加一個公共構造函數，該構造函數將對象中每個非 transient 字段的值作為參數，并在沒有任何驗證的情況下將值存儲在字段中？如果沒有，則必須提供 readObject 方法，并且它必須執行構造函數所需的所有有效性檢查和防御性復制。或者，你可以使用序列化代理模式（[Item-90](/Chapter-12/Chapter-12-Item-90-Consider-serialization-proxies-instead-of-serialized-instances.md)）。強烈推薦使用這種模式，否則會在安全反序列化方面花費大量精力。
There is one other similarity between readObject methods and constructors that applies to nonfinal serializable classes. Like a constructor, a readObject method must not invoke an overridable method, either directly or indirectly (Item 19). If this rule is violated and the method in question is overridden, the overriding method will run before the subclass’s state has been deserialized. A program failure is likely to result [Bloch05, Puzzle 91].
readObject 方法和構造函數之間還有一個相似之處，適用于非 final 序列化類。與構造函數一樣，readObject 方法不能直接或間接調用可覆蓋的方法（[Item-19](/Chapter-4/Chapter-4-Item-19-Design-and-document-for-inheritance-or-else-prohibit-it.md)）。如果違反了這條規則，并且涉及的方法被覆蓋，則覆蓋方法將在子類的狀態反序列化之前運行。很可能導致程序失敗 [Bloch05, Puzzle 91]。
To summarize, anytime you write a readObject method, adopt the mindset that you are writing a public constructor that must produce a valid instance regardless of what byte stream it is given. Do not assume that the byte stream represents an actual serialized instance. While the examples in this item concern a class that uses the default serialized form, all of the issues that were raised apply equally to classes with custom serialized forms. Here, in summary form, are the guidelines for writing a readObject method:
總而言之，無論何時編寫 readObject 方法，都要采用這樣的思維方式，即編寫一個公共構造函數，該構造函數必須生成一個有效的實例，而不管給定的是什么字節流。不要假設字節流表示實際的序列化實例。雖然本條目中的示例涉及使用默認序列化形式的類，但是所引發的所有問題都同樣適用于具有自定義序列化形式的類。下面是編寫 readObject 方法的指導原則：
- For classes with object reference fields that must remain private, defensively copy each object in such a field. Mutable components of immutable classes fall into this category.
對象引用字段必須保持私有的的類，應防御性地復制該字段中的每個對象。不可變類的可變組件屬于這一類。
- Check any invariants and throw an InvalidObjectException if a check fails. The checks should follow any defensive copying.
檢查任何不變量，如果檢查失敗，則拋出 InvalidObjectException。檢查動作應該跟在任何防御性復制之后。
- If an entire object graph must be validated after it is deserialized, use the ObjectInputValidation interface (not discussed in this book).
如果必須在反序列化后驗證整個對象圖，那么使用 ObjectInputValidation 接口（在本書中沒有討論）。
- Do not invoke any overridable methods in the class, directly or indirectly.
不要直接或間接地調用類中任何可被覆蓋的方法。
---
**[Back to contents of the chapter（返回章節目錄）](/Chapter-12/Chapter-12-Introduction.md)**
- **Previous Item（上一條目）：[Item 87: Consider using a custom serialized form（考慮使用自定義序列化形式）](/Chapter-12/Chapter-12-Item-87-Consider-using-a-custom-serialized-form.md)**
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- Chapter 2. Creating and Destroying Objects（創建和銷毀對象）
- Item 1: Consider static factory methods instead of constructors（考慮以靜態工廠方法代替構造函數）
- Item 2: Consider a builder when faced with many constructor parameters（在面對多個構造函數參數時，請考慮構建器）
- Item 3: Enforce the singleton property with a private constructor or an enum type（使用私有構造函數或枚舉類型實施單例屬性）
- Item 4: Enforce noninstantiability with a private constructor（用私有構造函數實施不可實例化）
- Item 5: Prefer dependency injection to hardwiring resources（依賴注入優于硬連接資源）
- Item 6: Avoid creating unnecessary objects（避免創建不必要的對象）
- Item 7: Eliminate obsolete object references（排除過時的對象引用）
- Item 8: Avoid finalizers and cleaners（避免使用終結器和清除器）
- Item 9: Prefer try with resources to try finally（使用 try-with-resources 優于 try-finally）
- Chapter 3. Methods Common to All Objects（對象的通用方法）
- Item 10: Obey the general contract when overriding equals（覆蓋 equals 方法時應遵守的約定）
- Item 11: Always override hashCode when you override equals（當覆蓋 equals 方法時，總要覆蓋 hashCode 方法）
- Item 12: Always override toString（始終覆蓋 toString 方法）
- Item 13: Override clone judiciously（明智地覆蓋 clone 方法）
- Item 14: Consider implementing Comparable（考慮實現 Comparable 接口）
- Chapter 4. Classes and Interfaces（類和接口）
- Item 15: Minimize the accessibility of classes and members（盡量減少類和成員的可訪問性）
- Item 16: In public classes use accessor methods not public fields（在公共類中，使用訪問器方法，而不是公共字段）
- Item 17: Minimize mutability（減少可變性）
- Item 18: Favor composition over inheritance（優先選擇復合而不是繼承）
- Item 19: Design and document for inheritance or else prohibit it（繼承要設計良好并且具有文檔，否則禁止使用）
- Item 20: Prefer interfaces to abstract classes（接口優于抽象類）
- Item 21: Design interfaces for posterity（為后代設計接口）
- Item 22: Use interfaces only to define types（接口只用于定義類型）
- Item 23: Prefer class hierarchies to tagged classes（類層次結構優于帶標簽的類）
- Item 24: Favor static member classes over nonstatic（靜態成員類優于非靜態成員類）
- Item 25: Limit source files to a single top level class（源文件僅限有單個頂層類）
- Chapter 5. Generics（泛型）
- Item 26: Do not use raw types（不要使用原始類型）
- Item 27: Eliminate unchecked warnings（消除 unchecked 警告）
- Item 28: Prefer lists to arrays（list 優于數組）
- Item 29: Favor generic types（優先使用泛型）
- Item 30: Favor generic methods（優先使用泛型方法）
- Item 31: Use bounded wildcards to increase API flexibility（使用有界通配符增加 API 的靈活性）
- Item 32: Combine generics and varargs judiciously（明智地合用泛型和可變參數）
- Item 33: Consider typesafe heterogeneous containers（考慮類型安全的異構容器）
- Chapter 6. Enums and Annotations（枚舉和注解）
- Item 34: Use enums instead of int constants（用枚舉類型代替 int 常量）
- Item 35: Use instance fields instead of ordinals（使用實例字段替代序數）
- Item 36: Use EnumSet instead of bit fields（用 EnumSet 替代位字段）
- Item 37: Use EnumMap instead of ordinal indexing（使用 EnumMap 替換序數索引）
- Item 38: Emulate extensible enums with interfaces（使用接口模擬可擴展枚舉）
- Item 39: Prefer annotations to naming patterns（注解優于命名模式）
- Item 40: Consistently use the Override annotation（堅持使用 @Override 注解）
- Item 41: Use marker interfaces to define types（使用標記接口定義類型）
- Chapter 7. Lambdas and Streams（λ 表達式和流）
- Item 42: Prefer lambdas to anonymous classes（λ 表達式優于匿名類）
- Item 43: Prefer method references to lambdas（方法引用優于 λ 表達式）
- Item 44: Favor the use of standard functional interfaces（優先使用標準函數式接口）
- Item 45: Use streams judiciously（明智地使用流）
- Item 46: Prefer side effect free functions in streams（在流中使用無副作用的函數）
- Item 47: Prefer Collection to Stream as a return type（優先選擇 Collection 而不是流作為返回類型）
- Item 48: Use caution when making streams parallel（謹慎使用并行流）
- Chapter 8. Methods（方法）
- Item 49: Check parameters for validity（檢查參數的有效性）
- Item 50: Make defensive copies when needed（在需要時制作防御性副本）
- Item 51: Design method signatures carefully（仔細設計方法簽名）
- Item 52: Use overloading judiciously（明智地使用重載）
- Item 53: Use varargs judiciously（明智地使用可變參數）
- Item 54: Return empty collections or arrays, not nulls（返回空集合或數組，而不是 null）
- Item 55: Return optionals judiciously（明智地的返回 Optional）
- Item 56: Write doc comments for all exposed API elements（為所有公開的 API 元素編寫文檔注釋）
- Chapter 9. General Programming（通用程序設計）
- Item 57: Minimize the scope of local variables（將局部變量的作用域最小化）
- Item 58: Prefer for-each loops to traditional for loops（for-each 循環優于傳統的 for 循環）
- Item 59: Know and use the libraries（了解并使用庫）
- Item 60: Avoid float and double if exact answers are required（若需要精確答案就應避免使用 float 和 double 類型）
- Item 61: Prefer primitive types to boxed primitives（基本數據類型優于包裝類）
- Item 62: Avoid strings where other types are more appropriate（其他類型更合適時應避免使用字符串）
- Item 63: Beware the performance of string concatenation（當心字符串連接引起的性能問題）
- Item 64: Refer to objects by their interfaces（通過接口引用對象）
- Item 65: Prefer interfaces to reflection（接口優于反射）
- Item 66: Use native methods judiciously（明智地使用本地方法）
- Item 67: Optimize judiciously（明智地進行優化）
- Item 68: Adhere to generally accepted naming conventions（遵守被廣泛認可的命名約定）
- Chapter 10. Exceptions（異常）
- Item 69: Use exceptions only for exceptional conditions（僅在確有異常條件下使用異常）
- Item 70: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors（對可恢復情況使用 checked 異常，對編程錯誤使用運行時異常）
- Item 71: Avoid unnecessary use of checked exceptions（避免不必要地使用 checked 異常）
- Item 72: Favor the use of standard exceptions（鼓勵復用標準異常）
- Item 73: Throw exceptions appropriate to the abstraction（拋出能用抽象解釋的異常）
- Item 74: Document all exceptions thrown by each method（為每個方法記錄會拋出的所有異常）
- Item 75: Include failure capture information in detail messages（異常詳細消息中應包含捕獲失敗的信息）
- Item 76: Strive for failure atomicity（盡力保證故障原子性）
- Item 77: Don’t ignore exceptions（不要忽略異常）
- Chapter 11. Concurrency（并發）
- Item 78: Synchronize access to shared mutable data（對共享可變數據的同步訪問）
- Item 79: Avoid excessive synchronization（避免過度同步）
- Item 80: Prefer executors, tasks, and streams to threads（Executor、task、流優于直接使用線程）
- Item 81: Prefer concurrency utilities to wait and notify（并發實用工具優于 wait 和 notify）
- Item 82: Document thread safety（文檔應包含線程安全屬性）
- Item 83: Use lazy initialization judiciously（明智地使用延遲初始化）
- Item 84: Don’t depend on the thread scheduler（不要依賴線程調度器）
- Chapter 12. Serialization（序列化）
- Item 85: Prefer alternatives to Java serialization（優先選擇 Java 序列化的替代方案）
- Item 86: Implement Serializable with great caution（非常謹慎地實現 Serializable）
- Item 87: Consider using a custom serialized form（考慮使用自定義序列化形式）
- Item 88: Write readObject methods defensively（防御性地編寫 readObject 方法）
- Item 89: For instance control, prefer enum types to readResolve（對于實例控制，枚舉類型優于 readResolve）
- Item 90: Consider serialization proxies instead of serialized instances（考慮以序列化代理代替序列化實例）