合成和指定修饰符的区别很简单——指定修饰符是隐式声明的，而合成修饰符既没有隐式声明也没有显式声明。关于这个问题有很好的文章，Java规范对早期引入到Java中的合成修饰符也有详细的说明。以下关于合成修饰符的详细信息是从那些文章中提取出来的，因为它们包含了完整的细节。请在末尾找到参考资料。
The Synthetic attribute is a fixed-length attribute in the attributes
table of a ClassFile, field_info, or method_info structure (§4.1,
§4.5, §4.6). A class member that does not appear in the source code
must be marked using a Synthetic attribute, or else it must have its
ACC_SYNTHETIC flag set. The only exceptions to this requirement are
compiler-generated methods which are not considered implementation
artifacts, namely the instance initialization method representing a
default constructor of the Java programming language (§2.9), the class
initialization method (§2.9), and the Enum.values() and Enum.valueOf()
methods. Java synthetic classes, methods and fields are for java
runtime’s internal purposes. We may not need to have knowledge about
them to write the code.
The Synthetic attribute was introduced in JDK release 1.1 to support
nested classes and interfaces.
The Synthetic attribute has the following format:
Synthetic_attribute {
u2 attribute_name_index;
u4 attribute_length;
}
The items of the Synthetic_attribute structure are as follows:
attribute_name_index The value of the attribute_name_index item must
be a valid index into the constant_pool table. The constant_pool entry
at that index must be a CONSTANT_Utf8_info (§4.4.7) structure
representing the string "Synthetic".
attribute_length The value of the attribute_length item is zero. Uses
of Java Synthetic • It might be useful in debugging sessions, when we
see those synthetic stuff in stack trace we can understand what it is.
• AOP, generics, enums uses Java synthetic. • Java reflection API
exposes method to check if an element is synthetic. • A regular java
application programmer will not require synthetic for day to day
programming. • This knowledge may be required in interviews but that
doesn’t mandate that you will use it in the project. When synthetic is
created? When an enclosing class accesses a private attribute of a
nested class, Java compiler creates synthetic method for that
attribute. If there is a getter method available in source then this
synthetic method will not be created. Similarly for constructor of
inner classes also synthetic is created. There are many occasions,
like this where a synthetic field or method or class is created.
The opens package was implicitly declared in the source of the module
declaration. This dependence was declared in the module declaration. A
mandated construct is the one that is not explicitly declared in the
source code, but whose presence is mandated by the specification. Such
a construct is said to be implicitly declared. One example of a
mandated element is a default constructor in a class that contains no
explicit constructor declarations. Another example of a mandated
construct is an implicitly declared container annotation used to hold
multiple annotations of a repeatable annotation type. Ex:
Module claim
requires mandated java.base
Line 1. Defines the module called claim. In line 2 defines every
module depends on java.base module except java.base. That means export
was implicitly declared in the source module declaration.
module m { }并使用javap -v检查生成的module-info.class，则会看到requires java.base设置了ACC_MANDATED标志。较难找到设置了ACC_SYNTHETIC的模块示例，但代理类被封装时是一个例子。在这种情况下，生成的动态模块具有设置了ACC_SYNTHETIC标志。 - Alan Bateman