Still confused about Android Binder? This article will help you understand it in seconds!

Android Binder is one of the most core IPC (inter-process communication) mechanisms in the Android system. It builds an efficient and secure bridge between Android applications and system services. This article will deeply analyze the working principle of the Binder mechanism to help everyone understand its core concepts and implementation methods.

1. Why do we need Binder?

In the Android system, applications run in independent processes and cannot directly share memory with each other. Inter-process communication (IPC) is an essential means to implement system functions, such as:

1. Applications manage lifecycles through ActivityManagerService
2. Access MediaServer for audio and video playback
3. Screen rendering through SurfaceFlinger

Traditional IPC methods (such as Socket, shared memory, pipes, etc.) are either complex or inefficient. Binder, as an Android-specific IPC mechanism, has the following advantages:

✅ Efficient: Based on single copy design, avoid additional data copying between processes

✅ Security: Ensure the legitimacy of communication through UID/PID mechanism

✅ Unification: Integrate the driver layer and the user layer to provide a consistent API

2.Binder's core architecture

The Binder mechanism consists of four key parts:

1️⃣ Client: The process that initiates the request, such as App calling system services

2️⃣ Server: The process that provides services, such as AMS, WMS, etc.

3️⃣ Binder driver (kernel layer): responsible for managing the Binder thread pool, message delivery, and permission checking

4️⃣ ServiceManager: used to register and query Binder services

Binder Architecture Diagram

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3. Binder communication process

When an application calls a system service, the complete process of the Binder mechanism is as follows:

Step 1: The client obtains the Binder proxy

Query the Binder proxy (BpBinder) of the target service through ServiceManager

Step 2: Request data encapsulation and transmission

The client encapsulates the request data into Parcel and then sends it to the server through the Binder driver.

Step 3: The server processes the request

After the target service process receives the request, the Binder thread pool unpacks the Parcel and executes the corresponding business logic

Step 4: Return the results

After processing is completed, the Parcel result is returned and the driver sends the data back to the client

4. Analysis of Binder key components

In the Binder mechanism, there are multiple core components involved. Let's analyze them in detail:

(1) Binder Proxy and Binder Native

When Binder is passed between processes, it has two roles: Proxy and Native:

• BpBinder (Binder Proxy): runs on the client and is actually a proxy for remote objects
• BBinder (Binder Native): runs on the server side and represents the real service object

📌 Example code:

 class MyService : public BBinder { public: status_t onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags) override { ALOGD("Received Binder call: %d", code); reply->writeString16(String16("Hello from Service!")); return NO_ERROR; } };

(2) Parcel: Efficient Data Encapsulation

Parcel is responsible for serializing and deserializing data to avoid redundant copies. It is similar to the C++ Parcel class and transfers data through write*() and read*() methods.

📌 Example: Parcel transfer string

 Parcel data, reply; data.writeString16(String16("Hello Binder")); binder->transact(1, data, &reply, 0); String16 response = reply.readString16();

(3) Binder driver: kernel communication bridge

Binder is implemented in the Linux kernel as binder.c. Its main functions include:

✅ Thread management (Binder thread pool)

✅ Inter-process data transfer

✅ Permission verification (based on UID/PID)

The kernel uses the ioctl mechanism to process Binder requests, for example:

 ioctl(binder_fd, BINDER_WRITE_READ, &bwr);

5. Binder thread pool and message scheduling

The Binder thread pool is an important mechanism on the server side. It manages multiple Binder threads to process IPC requests and improve concurrency capabilities.

📌 Key Features:

• Dynamic expansion: When requests increase, the thread pool can automatically expand
• Thread reuse: avoiding the overhead of creating and destroying threads
• Message queue: waitForResponse() to wait for requests and process them

6. Summary and reflections

As the core IPC mechanism of Android, Binder has become the cornerstone of system service communication with its high efficiency, flexibility and security. Its core includes:

✔ Binder Proxy and Native to implement remote calls

✔ Parcel provides efficient serialization

✔ Binder driver is responsible for message delivery

✔ Thread pool improves concurrency

💡 Future Exploration:

• Application of Binder in AIDL
• Comparison between HIDL and AIDL (after Android 8.0, HAL migrated to HIDL)
• In-depth research on Binder process isolation and security

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